Because we don’t have good youth institutes yet. You look at American football,baseball,basketball all of them are high school sports with state playoff etc “soccer” is not that big here yet. It’s getting there slowly but it still lags behind the other sports. Especially in college scholarship/money part too. Maybe one day it will be as big.
Title IX (college sports equality for the sexes) helped women’s soccer before it was a global sport for women, thus giving it the springboard needed to be an early success, which led to a pro league that attracted the best women from around the world. Men’s soccer, on the other hand, has had to rely on the 18-22 yr olds coming through the college ranks. In the rest of the world, those players would be full time pros at a club, rather than in a very part time set up, as found in college, but there was not even a high quality pro league, let alone the youth set ups needed for success.
I was an English high school kid who had played at a Man City, Sunderland, Leeds United and Oldham Athletic before I went to the USA on a college scholarship. I played on a team with international players (Ireland, England, Ghana, Canada, Jamaica) and USA National Youth Team players that was 2nd in the nation when they recruited me. That was 1982, so I’d say there has been a pretty vibrant US soccer presence since forever.
The key has always been money. The US sports calendar is stuff full of other sports, which is why the US soccer season is different from the rest of the world...TV money!!! It is impossible to compete on the weekends in the Fall (American football, both college on Sat and NFL on Sun), which then runs into the hockey and basketball seasons in the winter, which then runs into the Baseball season which runs from April to October. The decision to run soccer up against the baseball season is one of TV coverage availability and thus sponsorship and TV revenues. In fact, when MLS first started, the league used to PAY to be on TV.