Whether you value the trait or not, we put an emphasis on raw athleticism and physical fitness in almost all American professional sports. The average human isn't fully physically mature until 21 or so. That's typically when a kid is ready for professional play after the high school and college athletics experience.
In the European football paradigm, players should be turning pro by the end of their secondary educations, around 17-19. If a player hasn't made it as a professional by 21-23, his typically seen as lost cause.
We tend to get more athletic and durable players, who develop the technical abilities later than that of European football academy prospects.
This post was edited on 3/4 at 4:39 pm