Why is it acceptable to resign?

Many of the other answers haven't sufficiently highlighted the key point here:

Defeat in chess can be theoretically inevitable

The critical reason it's acceptable to resign in chess and not in athletic competitions is that there is always the theoretical possibility of a comeback in an athletic sport and its probability is always, hypothetically, a function of the trailing side's skill. If an U13 youth team losing badly in its soccer game were to suddenly start playing like Real Madrid - we see it in the movies all the time! - they could probably salvage several goals within a few minutes. This isn't the case in chess. No force on earth can save you once the endgame is theoretically winning for your opponent, or in another equally clear-cut scenario where you have blundered heavy material. Swapping in Carlsen won't help. Swapping in Stockfish won't help. Swapping in a God algorithm will not help. At least it shouldn't - if your opponent can close it out.

Waiting for a blunder is considered unsportsmanlike

Playing on in the sole hope that your opponent will blunder is perceived as unsportsmanlike. In particular, we've established that you've reached a position where saving the game is impossible no matter how well you play. Once you acknowledge that your opponent, playing as they currently do or based on your knowledge of them, should be able to win from their current position even against perfect play, then it becomes clear your intention in continuing the game is to hope for a blunder. This attitude is perceived as unsportsmanlike across all competitive events: I believe that aspiration to perform well yourself, not for your opponent to slip up, is considered a cornerstone of sportsmanship.

Playing on wastes your time, energy, and morale

Chess may be a somewhat equal game from the starting position, but once at a heavy material disadvantage - even if your opponent can't prove mate is imminently forceable - the flavour of gameplay is an inherently unequal and 'unfair' one. This affects every action taken in the game, every plan and decision made, in a way that must embrace that inequality (for example, Staunton considered it inadvisable for a player giving Queen odds to exchange their Knight for a Rook, as it would leave too little remaining material for them, although in an even game you'd usually be delighted to be an exchange up).

Since this requires a different type of play from an equal game, and inevitably a less pleasant one, morale tends to take a turn for the worse defending positions that are almost inevitably lost because of an opponent's oppressive material advantage. Athletic competitions generally do not have this factor: how you play for a win if two sets down in tennis is really just the same as how you play to win from the start, but in chess you often must adopt a defensive, tricky, or swindling playstyle if even two pawns down against a strong opponent. 'Contempt' was developed for chess engines for this very reason: even the most objective agents underperform in material odds scenarios if they try to play chess when disadvantaged as if that disadvantage didn't exist.

Board games obey these criteria

Let's consider Monopoly, Risk, etc. In these I would say that gameplay changes, like in chess, if your position is greatly disadvantaged. However, if resignation is indeed unusual - and I'm not sure it is - there are a number of reasons why this may be so that don't apply to chess:

1. When evaluating the position as theoretically winning, you must consider whether with any combination of optimal play by the potential resigner and extreme luck, the position is still lost. This is a much more extreme condition than typically resignable positions in chess because luck is a fundamental part of these games, so if for example you own only one property, but by luck your opponents could keep landing on that property and losing all their money to you, a win is far from theoretically impossible for you.
2. Resignation requires a group decision to acknowledge a single player as winner. Otherwise you must decide what to do with the properties or countries of the resigned individuals.
3. There is no need to conserve time, energy, or morale for future games in casual settings, as you do not have further imminent games to play, as you would in a tournament setting (which is much more common for chess).

(started out as comments. now an answer.)

i like Ozzy08's analogy with points or rounds, but i take a different path: View chess as a round of games instead of just 1 game.

This view is supposed to address 2 things:

1. Many answers here seem to make chess like a sui generis with regards to resignations.

2. 'it is also considered poor behavior to leave the field of play before the game is complete, regardless of the score'.

It's like this:

1. For point 1, I notice similar things to resignation in other games. for example csgo: people don't resign whole games similar to, say, basketball or soccer/football/rugby, but they do 'resign' rounds.

   • eg if a team is about to lose the current round, then depending on the money or guns that players have, they might consider to save whatever guns or money they have. in this way it's kind of like a 'resignation' for the round.

   • (I recall something similar for quarters in basketball from Kuroko no Basuke. Apparently when one team is dominating a particular quarter, it's hard to be un-dominated for the quarter. So you just make up in the next quarter. But I'm not familiar enough with basketball to confidently make this analogy.)

2. For point 2, under this view, we should compare 1 game of, say again, basketball, soccer/football/rugby or csgo, not to 1 round of chess but rather to 1 match of several rounds of chess.

   • eg in the FIDE World Fischer Random Chess Championship 2019, it makes sense for magnus carlsen to resign against wesley so in any 1 particular round, but it doesn't make sense to resign the entire championship match after being behind 10.5-1.5 at the end of the slow rapid rounds.

You might ask 'What if I were completely new and were playing 11 games against Wesley So, and I've already lost 5 games? Now, may I resign the whole match?' but then it's the same thing, or at least similar I believe, as a completely new basketball player playing against a pro: Resign in the wesley so case if and only if you resign in the basketball case.

3. In re Brian Towers♦' war analogy, I view resignation not as surrendering the war but as conceding a battle.

Here is something interesting in chess, different from any other conventional sports, i.e., here resigning doesn't lead to poor-sportsmanship of the player, rather it's the intellectual advantage of the opponent, that there's nothing can be done for the player. And, also the term Sportsmanship, is, I believe ill-defined in chess. For a game like chess, which is purely the "Battle of intellect", doesn't come along with, regular sport-like attributes, which we find in any other sports. Thus, the "resigning" simply mean, there's nothing can be done here, and the opponent is clearly winning. And, this is totally logical, not to waste any much time. And this very well suits, in chess.

Some games, you can't just give up. For example, in a football match, there are spectators who want to see some sport, there are players who may enjoy the exercise, and there is also a question of the degree of victory counting for something. (e.g. goal ratio may act as a tie-breaker in the league.)

In chess on the other hand, the value of resignation even in a situation where loss is not certain can be positive. In a casual game it can allow the players to start another game. In a more serious setting it allows the losing player to conserve energy and morale, to show respect for the winning player, and take a break perhaps to focus on the next match.

Chess contains some beautiful draw mechanisms: stalemate, 50 move rule, draw by repetition, which serve to keep interest in the game, and (together with the clock) put pressure on the winning player to actually win. Chess is a subtle game and many resignations are mistaken, in that a draw or even a win is still possible.

Concessions in other sports...

In multi-game cricket matches, a captain may essentially stop scoring points for his side, in order to allow more time to bowl out the opponents. This is quite specialized, but it can result in the fans being sent home earlier than they would have been otherwise.

Golf is an interesting one. In "match play" (essentially knock-out 1-1 matches) it is normal and polite players to concede trivial putts (maybe less than 50cm?) to the opponent. However in stroke play, where the folk going round the course together are not competing with one another directly, then no putt concessions are possible.

Snooker is again interesting. It's usually a 1-1 knock-out format, and it's common for a player who is far behind in the score to concede at the point when their opponent has just ended his turn. It would be very impolite to concede at any other point, because it potentially stops the player from being able to complete a long break. Indeed it goes so far that if there is just White & Black ball on the table, then a player effectively claims the game, assuming his opponent's resignation.

Finally, in backgammon, you can only resign in response to being offered to double the stakes. This is a key strategic issue, which forces the timing of doubles, as one doesn't want to double so late that the possibility of exacting a penalty from the opponent is too obvious to both players.

One of the main reasons could be that chess has no limit of time or "score" needed to win. The examples we see here where resigning is also good sportanship are often that kind of game (risk, real-time strategy videogames...)

The point is not making the game unnecessarely long when the result is already certain

Note that the core game of chess would be almost unchanged if it ended with the actual capture of a king rather than checkmate. In a sense, checkmate itself is a from of resignation, one that is written into the rules of the game. The game ends when the king would be lost on the next move. Resignation is just an extension of the logic of ending the game when the loss of the king becomes inevitable. Sometimes that inevitability becomes apparent more than one move in advance.

To follow up on Brian's answer about war, I think competitive Real Time Strategy games are a very close analogy.

For those unfamiliar: In these games players typically build up bases, then build armies, then those armies fight. If one army has a decisive where the victor has enough troops remaining, they go destroy the other person's base and all their straggling units. Once the opponent is 100% destroyed, the conqueror is declared victor. In competitive play, users typically say 'GG' (Good Game) and resign after they lose the critical battle. In fact, there is a often similar stigma of poor sportsmanship for not resigning, similar to chess.

The endgame here is often very boring. There isn't any hope of comeback, but it can take a while to burn down all the opponents buildings and hunt down all their straggling units. The endgame is qualitatively different than the gameplay to that point. It is merely waiting for buildings to burn down. There is no skill involved. This isn't fun for the loser and usually not fun for the winner either. It just wastes people's time - and this can be a substantial amount of time, depending on the game.

One sided chess endgames are similar. There isn't much skill or thought left. Its just going through the motions, and those motions have little in common with normal chess - even normal chess endgames. If you're up a two queens, it doesn't matter if you have the wrong rook pawn or you don't move your king up in the endgame or hang a queen, even.

This differs from soccer, where the gameplay is still effectively the same at the end of a 10-0 defeat as it is at the start of a 1-1 tie. There may be some strategic and tactical changes to the approaches of both sides, but the core skills and gameplay mechanics are constant. Losing 10-0 sucks, but you're still playing soccer. You are playing soccer (presumably) because you like playing soccer. You still get to do something you enjoy, even in defeat.

In closing, there are two reasons we play games: we care about the result and we care about the gameplay intrinsically. If the result is not in question and the gameplay changes to become bad, why keep playing? Why force someone else to keep playing?

Why is it acceptable to resign?

Chess has its origins as a war game. It was used to model war and perhaps to play a part in the training of generals. In war it is standard practice to surrender when it is clear that defeat is unavoidable. This is an obvious humane thing to do to minimize loss of life. Crossing over into chess as the model of war this translates into resignation.

Most sports have a numerical end. A game of basketball ends after 4 quarters and the team with the highest score wins and a game of tennis, which does not have a time constraint, is decided when a player wins the best out of three sets. To end a game of chess, however, requires either checkmate or stalemate. Unlike other sports, there is no linear path towards these such as scoring the most amount of points. A game of chess can end as early as move 1 or as late as move 200. Since most sports run on a clock, ending them before the allotted time would be both humiliating to the opposing team and to the fans who paid to watch the game, even if there is no chance that the losing team can come back, because the termination of the event symbolizes that the opposing got beaten so badly that the game had to be ended early. Also, the reason why a soccer team who is losing 10-0 don't just walk off the field is partly to avoid humiliation but also financial. Would anyone like to see team that quits when they are losing? What does that say about the team's morale? However, culture also plays a big role. Since resignation has been in place for so many years and the practice of resignation has never been implemented in traditional sports, it has stayed that way as there is no practical reason to change it.

In chess, this humiliation does not carry on, because when you reach a high enough level, it is assumed that your opponent won't make silly mistakes and that it is therefore understandable to end the game early. Playing on might be seen as unsportsmanlike because you are conveying the message that you believe that your opponent is not good and that he will blunder in a completely winning position. Furthermore, the winning player might feel like his opponent is purposely playing on in order to annoy him and waste his time, even if this is not the case. This is not to say that once you are inferior you should resign, you should always look for chances to complicate the position in hopes that you can gain an advantage. However, when there is no possibility for counterplay, playing on is a waste of time for both players.