I'm not going to get into your discussion about NBA players (aka the best of the best from Europe). I'm just going to explain to you why I think your broad, overarching title of "How are white Americans so bad at basketball?" is pretty silly.

The average white American male would wipe the floor with the average European male in a proper game of basketball. This has nothing to do with being athletically superior or anything. I say this simply because we are taught at a very young age the basic mechanics of basketball through recreational time at school (or, for some, through organized basketball). They know how to dribble, the basic mechanics of shooting, and, of course, the basic rules.

The average European male, however, knows absolutely none of that. Dribbling all over the place (likely illegally), hoisting up comical shots, breaking all the rules. As you've said, it is a niche sport so very few people know how its properly played and even fewer can do it themselves.

I don't say this from out-of-the-blue. I say this from experience. I've been on a mission trip to Eastern Europe and literally played pickup basketball in Hungary and Romania. I'm the most average-looking, 5'11", 155-pound dude out there and I was looking like LeBron.
Now, is that a small sample size? Yes. But I'd be willing to bet that I would have the same experience throughout most of Europe, except for very specific communities in which basketball is thriving.