In a word, money.
Bowl games came into existence because of money.
Conferences came into existence because of money.
The Iron Bowl ceased to be played for four decades because of money.
Mercer University - a small, private school in Macon, GA - can play Auburn and Alabama and walk off with a minimum of $3 million guaranteed ($1.5M each game). That can go a LONG way at a school of 8,600 students, particularly when they aren't paying $7 million a year to a head football coach.
I "get" the frustration with it. But it's a sort of way to redistribute wealth, a sort of "contribution to education" elsewhere. I won't go to the Mercer or Citadel of WCU games because of the travel cost, but a lot of people only get to see Alabama in person through such games, too.
It's a scrimmage that counts and besides - we have losses on our ledger to La Tech (twice), Central Florida, Northern Illinois, and ULM. There once was a time it wasn't the guarantee for us it is now.