The devil you know vs. the devil you don't?
OSU and Michigan account for 17% of the schedule and he is responsible for that. The HC also has responsbilities beyond just those two games.
From the perspective of a hiring/firing manager, do you make a decision - and a multi-million dollar one at that - based on 17% of an annual football schedule; a percentage of obligations that decrease when you add in all the other tasks the football coach must deal with on a yearly basis?
Edit - and if you do, wouldn't you have to guarantee that your new coach addresses the 17% that the current guy has failed to address and everything else that the position requires? From the perspective of a hiring/firing manager, that is actually a very daunting consideration.
It's not that complicated; if you are fine with how things are and think they will continue to be that way (to include support from fans/alums, ticket sales, donations, etc.), you don't do anything. If you think you can do better, and it is worth it to do so, you move on. How long will fans support a stagnate program? Not like we have a choice.