Do you really see American 18+ year olds taking time out of their college day to go do a grueling track workout, if it were not part of the school experience? Yeah, they like to hit the gym for 20 minutes and socialize, but Americans really don't think like the rest of the world. The only reason we have so many college age kids into track is precisely because of college. And where there's large numbers, you are going to find some champions. Plus some athletes are late bloomers, and they won't have incentive to spend all that time at the club, just to see if they will turn out good. In college you can be average and still have lots of fun. Club track isn't so much fun if you are always an also-ran.