The NFL isn't actually very good at coaching in general. It's very much a copycat league that occasionally understands why things work after they've been effective.
You have to make the offense around your QB, you can't really do that with a rookie as you don't really know what they can do. It's further complicated by teams drafting QBs that have no hope of fitting in with what a team currently does.
There's probably a lot of coaches that would be amazing in the NFL that never get a shot because the league is so incestuous at the moment.