It's the college system. The United States is wedded to the idea that everyone that wants to go to college should. The soccer pay to play system we have built up is therefore directed at building college athletes. Some exhibits:
-the girls, unlike the boys, don't have a clear high level academy tier...ECNL is very broad and the play is irregular throughout the league...some teams/players are the caliber to be selected for the USWNT, some aren't. Talent is diffused so the best aren't playing with and against the best.
-we expect the players to keep up their academics. Practices are generally limited (except at the highest levels) to two times a week after school, whereas the European academies are training 5 days a week including during school hours. The players in Europe don't go to college showcases, but to play in high level tournaments against teams from other countries.
-European players are all in on athletics and if they flame out, getting back on the academic track is very hard (US colleges are actually a fallback for the well off). The European academic track doesn't have everyone going to college (there is a rigorous trade school practice which is limited in the US) and entrance to college is exam (or influence) based. Here, players divide their attention between athletics and academics.
-the academies aggregate coaching talent and train them according to a system. In US clubs, the team is a little fiefdom run by the coach under the loose guidance of the club. Each team is like a box of chocolates....you don't know what you are going to get.
-it's pay to play which acts as a barrier for the poorer kids to entry, because the function is to set up a platform to be seen by colleges, instead of a fully funded academy that will make its money back (at least in part) when the player turns pro.