That's certainly true; it's true in most other sports as well. That's why for example baseball players spend anywhere from 2 to 5 or 6 years in the minor leagu

That's certainly true; it's true in most other sports as well. That's why for example baseball players spend anywhere from 2 to 5 or 6 years in the minor leagues before they make the big club (which many never do). There's no reason to prefer that they spend that developmental time playing for free on a university's team instead of for peanuts on a minor-league professional club. And there are plenty of reasons to prefer the alternative.