I can see your point a little bit, but your logic is very flawed here. The ability to do the job and winning football games don't necessarily go hand in hand. Your ability to do a job has completely to do with your ability to do the tasks required of that position.
If a school hired Tim Brewster to be head coach of their football program, they can rest assured that he has the ability to run a football program. You know that he will have the ability to perform the required job tasks of a collegiate football head coach, because he's already done it for multiple seasons. Now, you have no idea if he's going to win games for you, but you know he can do the job tasks. Most people when hiring want a little more out of their hire than that, of course, hence, why people get canned all the time even though they can do the job.
Winning games gets more into HOW MUCH ability you have to be a head coach, not if you have the ability or not. And even then, you see all the time where coaches will win one place, "fail" at other places. It's not like their ability to be a head coach suddenly changed. Circumstances around them did though. But whether they won 10 games at one school, or 3 games at another school (or even 10 games one season, and 3 games another season at the same school), their qualifications for doing the job still stood.