There are, what, 32 teams in the NFL? So 96 total potential HC, OC, DC positions?
All of those positions are currently occupied by coaches who have already worked their way up in the NFL, so there's maybe... 8-10 positions that would realistically be open for a current college coach to move into?
The fact is, the vast majority of college coaches simply aren't able to move up to the NFL. Maybe a small handful each year. So while it's easy to say "If I were a coach, I'd just ditch college and move to the NFL" the truth is that's just not a realistic option for the vast majority of college coaches.
I'll add to that by saying, I'd pretty happily put up with all the bulljive of coaching in college for a million or more per year. I currently put up with a ton of bulljive in corporate America, for less than that.