The German-American players aren't coming up from school and university athletics, either.
Our domestic supply is 90% NCAA (university)... There are reasons to be wary of the academy system, especially how it serves the kids who don't rise to the top, but it delivers the goods.
This was the constant refrain in a number of articles about when/whether the US would ever punch at its weight in world football. Our player-development system is almost exclusively delivered by schools and universities.