Because Belichick was a head coach once, with the Cleveland Browns. I don't know if the Jets will be better off with Groh than Belichick. No one does. That's the risk you take when you elevate an assistant to the top job rather than dip into the pool of former head coaches. You never know what you're getting.
All coaches know football.
They have to. But knowing football and running a team aren't the same thing. If it were, Ray Handley would have been one of the greatest coaches in the history of the league instead of what he turned out to be once the Giants gave him the head job in 1991 - a complete and utter disaster. How much you know about football has as much to do with your success as a head coach as how much you know about grammar has to do with your success as a novelist. In both cases, what you know about character development means everything.