Non-USA superstars in the NBA seem to be much taller on average than superstars from the USA: What exactly is the reason?

Note: here I'll refer to the "world" game; there's no one World game style of course, but there are some things that tend to be more common in non-NBA leagues due to some rule differences and court differences. It's a generalization of limited value, but it simplifies the answer.

I think that the answer to this would be the difference in the US and World games, in part, and in particular trends in the US game in the last five years or so that have massively changed the job of some players.

Taller is better, certainly, so long as it's not correlated with worse aspects - such as ability to make shots. Lebron James is, well, Lebron James in part because of his height - at 6'9" he can shoot over the 6'4" players who otherwise have a similar skill set. He has the abilities of a guard in the body of a power forward, and thus is one of the best players of all time. Eat a little less veggies as a kid, and 6'6" Lebron James is maybe Paul Pierce good, but not Lebron James good.

That caveat, though, is pretty important: the likelihood of a taller player being as good a shot as a shorter player is not terribly good. That's not solely because height makes it hard to be good at basketball skills - it can make it harder to be coordinated, but the real problem is there are maybe a hundred people at most over 7' in the US; there are tens of thousands of people who are 6'6" (1/1000 of US men, so maybe as many as 150,000) and even more who are 6'3" (2% of US men, so over a million). Given that of those hundred, only a few will be in their 20s at any one point - maybe a dozen or so? - and it's unsurprising you don't often see a person 7' or taller that is particularly good at basketball skills beyond the height, while you have millions of people vying to be Steph Curry or James Harden. Of course you find better skilled players at that height!

World players tend to not be as well respected over here - that's slowly changing, but look at how hard it was to get Dirk Nowitzki to be respected here; when he started he was only drafted ninth, despite having a skillset that competed with adults, a nearly Lebron-level ability and at seven feet tall. This is changing, in no small part thanks to Dirk himself; but it's slow.

One of the reasons world players tend not to be as respected here is that they're thought to be soft - the NBA used to be a far more physical league, and still has that reputation, though it's not truly deserved anymore I don't personally feel. As such, taller players tend to have an easier time getting over here, because they're more likely to be able to survive the physical game than a smaller player. Think about the Isaiah Thomas Pistons in the 80s, or the Patrick Ewing Knicks; those were bruising, physical teams.

More importantly, though, the taller World players are also a good fit for the US game right now. The World game has always tended to place a higher value on basketball ability than physical ability; look at players like Dirk, who was 7'0" but had an amazing outside game. In 2020 that's not shocking or unique, but in 1998 it was not common - Kevin Garnett was the only truly direct comparison, really, over here, and he was unique in his own way. The fact that the US game has shifted so much recently towards shooting has a direct impact here - even the center needs to shoot; no longer are Garnett and Nowitzki unique, but nearly every team has a center who's happy to shoot the three. Thus, tall international players are comfortable shooting long distance, and come over here.

I suspect it's also in part that taller international players are more likely to be the stars of their local teams for years - and thus more likely to have a well rounded game; here, a 7' player is almost certainly going to end up on Kentucky or Duke or something and not have to carry the load, unless they're truly special, after high school, and even in high school odds are a player who will end up 7' is going to be on an AAU team with other highly skilled players. In Croatia, or Germany, or Italy, it's more likely they'll be on a club team for years with somewhat less skill around them - and thus develop ball handling and distance shooting skills of their own. This is just my supposition, though.

Finally; for more evidence of the NBA game change mattering, look at what this would've been like twenty years ago. Picked at random, 2002: Starters Kevin Garnett and Tim Duncan on the West were 6'11" each, plus Dirk Nowitzki and Peja Stojaković; the East had Dikembe Mutumbo, who was born in the Congo but went to college in the US, and two reserves at 6'10", Alonzo Mourning and and Jermaine O'Neal. Four American (ish), two European, and one African players. Lots near 7'. And for whatever reason no Shaq that year! In fact, only four years before, you'd have had a ton of 7 footers from the US - 1998 had Rik Smits (7'4"!!), Shawn Kemp (6'10"), Jayson Williams (6'9"), Antoine Walker (6'8") on the east alongside Dikembe, and Shaq(7'1"), Garnett(6'11"), David Robinson (7'1"), Tim Duncan (6'11"), Karl Malone (6'9'), Vin Baker (6'11") - the list goes on. No shortage of tall Americans - but not what you'd expect of the game today, for better or for worse; there's just no place for a big guy who can't hit a shot from anywhere much outside of the restricted area.

One more "guess" - a few major failures, starting with the Bulls' historically disastrous draft and the failure of Greg Oden a few years later, made front offices hesitant to draft big centers - foot problems and other issues seemed to keep cropping up, while 6'6" guys were much safer. Unfortunately, height and weight are both bad for your feet! International players have the same issue, but since they come over at a few years older and often fully grown bodies, the scouts can figure out which ones are less likely to get hurt.