1) How sure are you? How many American boys grow up playing soccer versus Croatian boys? I don't know the numbers, do you? Or are you just claiming knowledge that you don't really have?
2) Don't confuse quantity and quality. Even if you're right and we have a lot more soccer-playing boys than they do, what's the filtering mechanism? If an American boy was an athletic freak, which sport does he choose? Football or basketball. That's where the paycheck is. That's what young American boys grow up dreaming of playing professionally. In Croatia, it's soccer (with basketball probably a distant second). So athletically, Croatia's team is, quite likely, more talented than America's team even if it's drawing from a smaller pool.
3) The marginal difference between our team and theirs is not that big. We're really not too far from playing more competitively on a global stage.