Why did the Avengers not travel back in time to right before the Snap?

1. Time travel in the MCU does NOT change the past. Rather the traveller is visiting a duplicate timeline's present - which can go forward with alternative events. Thus resulting in multiple universes.

2. Given that fact. Fighting Thanos to stop a snap doesn't solve the Avengers' goal of obtaining the stones to do something in their own universe. It's possible but it is the maximum difficulty way to do it.

3. Going back to points where they knew the stones were EASY to obtain is what they did. And even then it did not go completely smoothly.

Let’s go through all of your points:

1. This isn’t much of an advantage, they know where all of the Stones are at various points anyway when they are less guarded and so easier to take.

2. As per the above they know exact times and places anyway so this isn’t really an advantage.

3. He may have been weakened but Thanos is still an incredible foe, and in IW he’s not even trying. In Endgame he goes in trying and he puts all of the Avengers on the ground and walks away smiling without even breaking a sweat. He only loses by Tony’s “trick”.

4. Maybe so but that’s not much of an advantage and in all of the places they went the Children of Thanos wouldn’t have been present anyway. Thanos only finds out because Nebula’s storage interferes with the 2014 Nebula’s.

5. Again if a Stone is completely unguarded that’s easier than taking on Thanos who still could call for backup from his armies in the time it takes to steal the Stones.

6. No they couldn’t have. Time travel does not work like that in MCU, this is established several times:

   Smart Hulk: WHY DOES EVERYONE THINK THIS? THAT’S NOT TRUE. IF YOU TRAVEL TO THE PAST, THEN THAT PAST HAS BECOME YOUR PRESENT, AND YOUR FORMER PRESENT HAS BECOME THE PAST, WHICH NOW CAN’T BE CHANGED BY YOUR NEW FUTURE!

   _{Avengers: Endgame}

There are three main points though. They needed the easiest shot they had to get the Infinity Stones without failing. That means with as few as possible fights and less risk. Thanos isn’t a pushover even weakened from Stormbreaker or the Snap. On top of that he has the Space Stone; if he wanted to he could just zip across to somewhere else and then Snap leaving the Avengers clueless to where he is and the trip pointless.

To top this off they only had enough Pym Particles for one round trip so failure was not an option. They couldn’t risk any of that so needed to grab the Stones whilst they were unoccupied with as little risk as possible.

Scott: These are Pym Particles. You can’t shrink to the Quantum Realm without them, and since Hank Pym got snapped out of existence, we can’t make any more. I told you. We’ve only got enough for one round trip each, no do-overs.

_{Avengers: Endgame}

Doctor strange went through millions of possibilities, found only one that worked. If grabbing the glove would have worked, he would have suggested it.