Earlier quoted context omitted.
Why not? (I ask this in hopes of being persuaded, not to defend a position.)
Because colleges have the stated goal of educating people. Its cynical (as the commenter above mentioned at the end) to join the college with no intention of actually getting educated. Its called a 'shill' or ringer - somebody you get from outside to represent you, that is not actually a member of your organization beyond the single task (sports).
And do you suppose that people are just born being really good at sports? Of course not, it takes thousands of hours of learning/education. If we're going to treat things like math and economics as academic subjects, then I don't see how football is any different.