I see your point that the athletes are the ones doing the work that creates this wealth, but you're missing who the real draw is from the fans' perspective, which is the schools. People tune in to watch Notre Dame or USC not the players on the field.
If the NFL created their own development league and tried to sell tickets or airtime, it would drive no more traffic than minor league baseball. The fact that the competition becomes a source of school and regional pride is what drives the revenue in NCAA football and basketball.