There are precious few HC jobs, so if you are say, John Fox, and you want to coach again and Chicago is your best option, then you go there knowing that they don't have a franchise QB. That sucks, relative to what other coaches have to work with like MM, Belichick, Tomlin, etc.
Those are two very different situations by which to come into a head coaching job. Fox may likely be on the treadmill that many of these head coaches are on. Once they finally draft the guy they have tabbed as the "franchise QB", the team is desperate for a turn-around, and you almost have to cast your lot with the young guy, unless you can get a stop-gap vet, which also may not work out. I'm not sure what took the Rams so long to try and find one, but by that point, Fisher had no shot of a turn-around.