The NFL is stocked with full grown men.
Along with them being full grown men they are the cream of the CFB crop.
The worst player on the worst NFL team would be a star in CFB.
NFL coaches don't work nearly as hard as college coaches because NFL players are consummate pros allowing the coaches to scout oppositions and form game plans.
NFL coaches can actually rely on their players to know the play book and push themselves.
Weekly game plans in the NFL are more about adjusting the system for their new opponent while college coaches are using new plays every week.
NFL coaches going CFB are overwhelmed by system implementation, scouting, watching highschool game tapes, making calls and visits to recruits.
The high school football pool is vastly larger than the mostly baren D1 CFB landscape.
Here are just a few examples of NFL coaches going CFB and failing:
Mike Sherman
Dave Wannstedt
Mike Shula