I don't think it's that deep. Boxing, Track, Baseball have plenty of minority athletes and don't have these restrictions because they're not big money makers for schools. NFL probably wouldn't draft 18 year olds with high school level coaching so they would have to enter some kind of minor league which doesn't currently exist. Basketball, I agree with you, it doesn't make sense there.
The problem is the NCAA is not the only problem in the equation. The major sports leagues benefit heavily from the free development systems the NCAA has. The NFL is the most egregious culprit since they literally do not have any sort of developmental system at all. The NFL uses college football the same way baseball uses its farm system except its free for them. That bs needs to stop especially given the violent nature of football. Also the NBA is very guilty too of this bs but at least they now have a developmental league. Its kind of weird though watching international players who have been making money since 14 years old playing professional basketball like Ricky Rubio and guys like Zion and RJ Barrett had to do a year of unpaid labor in college basketball.