I think the reason for that is the shortness of NFL careers, the average NFL career is only something like three years. So if a player spends only two years in the minors, they would be damaged goods by the time they would finally make it to the NFL.
True, but that is after 3-4 years in college. I knew some college players in some of my classes—they really should not have been in college. Some were barely literate (no kidding).
They would be prime minor league prospects IMO.