The reason why Japan/China produce many good MLB/NBA stars is because of the globalization of the MLB/NBA, especially when teams toured those countries and promoted the sport.
Do you think that every single European country is absolutely shit at baseball for some genetical reason? NO! It is because hardly anyone in Europe gives a fuck about baseball because it is barely promoted/shown on TV. Put a 'World Series' or 2 in the UK, and within 5-10 years time you'll see an influx of British baseball players.
Without globalization of a sport, the talent pool is extremely limited to only those that watch/play the sport. The next Babe Ruth may have already come and gone, but never have actually picked up a baseball bat in his life because he lived in Europe where most people couldn't even tell you 1/4 of the rules.
I agree with you to a certain degree: Countries like f.e. Germany or Great Britain suck at baseball because the game is played by just a handful of people. Otherwise they would at least play at a higher level, probably not the top level for several years, but they would improve. But Japan has been a baseball country for a long time and that wasn't because of MLB or some sort of promotion.
And even in Europe you have countries like the Netherlands, where soccer is clearly the #1 sport, but baseball and softball are very popular too without MLB really promoting the sport agressively over there.
But I agree pretty much with betallsports when it comes to NFL and Europe: You have a fan basis over here (especially in Germany, where most of the NFL Europe teams were based in the end, and seemingly also a large group of fans in the UK), who really love and understand the game and you probably would have no problem at all to sell out a superbowl.
But if you take these fans aside, the rest of the country doesn't care for Football or even Baseball at all. It's just another sport, which gets ignored by the majority.
And I completely understand if American fans want "their" league to host the final game in the US because of several reasons (revenue, tv schedule or just because the major American sports finals should also be played in the US).