I don't think the unstructured format directly contributes to the playing strength but rather attracts more player to play in a local club. Even in the town where I life with less than 100'000 people there are 10 clubs, 168 teams and nearly 3000 (mostly semi-professional) soccer player. Of course not all of them are young anymore but extrapolate this numbers to the population of a country it becomes a huge talent pool available for the major clubs.

And compared to the US there is a far more dense competition as any state has its own national league and on top are the Champions, Europe and Conference league. So every major soccer team plays in a national and a europene league at the same time and thus the players get of course much more routine.

But hey, we suck at baseball and basketball.