Can you see how having 4 other sports and soccer being a very distant 4th places siphons off the elite athletes though. You can't tell me that a country of 330 million wouldn't consistently field a better team than most of Europe and South America if it was far and away the most popular sport. But it's not, it's been kind of an afterthought the most elite athletes in the US have 4 other sports they tend to pick first.

The amount of time and effort elite basketball and baseball players put into their sport is pretty similar with camps and summer leagues. Football is a bit different because it's so physical so playing more than the standard season would wreck your body, though the guys bound for D1 programs certainly start working out like it in highschool.

It probably is the most popular women's sport in the US vs being 4-5th. There isn't really a women's version of American football of any size worth mentioning anyway, softball isn't popular, and the WNBA is niche at best. So soccer actually does pull the best women athletes to it in the US.

The US has 3 sports that are more popular than soccer, and one other that soccer is just now starting to catch. That's the real reason, the country's elite athletes are playing American Football, basketball, and baseball instead. Even hockey is siphoning off a fair amount.