I've asked that same question after reading about many multi-million dollar coaching hires. Perhaps the answer is: It's not Board members' money, and their Board tenure and compensation doesn't depend on the success of the coach and the enterprise, that is, the football program.
It doesn't? Consider that in Nebraska, when a Regent declined to take a position on whether Bo Pelini should be fired or not, saying that wasn't a decision for the Regents, he had someone jump into the race to oppose him, and was nearly defeated. I personally think that the only reason he was re-elected was that the losses to Wisconsin and Minnesota came after the election.
What you say is how it should be, but today the reality it that yes, the Regents' job security may in fact be affected by the performance of the football team.