Why did sports become naturally segregated?

It's just social and economic factors at work again. Think about the Jim Crow eras, and who would have access to what. And how White society accepted Black people into which sports, as well as what sports Black people would be able to get equipment for. Once the gears get set in motion, the rest is obvious. Look at countries where Black people are the majority and you'll see that they play all kinds of different sports. Obviously genetics make people different racially but not THAT different that it would really impact what you're talking about. As time goes on and society changes, it will even out, just watch and see.

its not like you MUST be AfricanAmerican or White to join the NBA, it just happens.

Same with every other sport.

For example, Japanese people have less muscular bodies than Americans, and that is fact and i mean no racism because i am Japanese myself. That is why asian countries don't make it too far into the World Cup.

Genetics - A given body shape which is determined by your genetics will give you an advantage in certain sports for example someone who has a genetic structure that gives them a long body and limbs with a dense bone structure would excel at basketball whereas somebody with less dense bone structure and less muscle mass, broader shoulders may excel at swimming.

AA's only play a few sports and dedicate all their time to them. Naturally, they'll have more athletes in those fields. Many of them also go all or nothing, while white athletes realize that they need to focus on schoolwork in case they don't make it pro. Thus, many black athletes go all or nothing and some make it, but most end up broke.