There’s no way being a head coach is more fun than being an assistant. Head coaches have so many more obligations for their time than any assistant or coordinator. I think a huge number of these guys realize they aren’t head coach material. A few eventually get the job once and fail but ultimately settle into career coordinator types.
I bet Vic Fangio is having more fun as eagles DC than he did as Denver HC.
You get your bag from the one HC job and take a beating in reputation and get back to what you are good at, like Steve Spagnuolo, probably making more as a coordinator than he did as HC long ago.