Also Kirk Ferentz should not be grouped with Carroll and you are giving Urban Meyer way too much credit for a guy who has always used the previous coaches players.
Ferentz is not at an elite program and isn't that highly regarded in NCAA football circles. Proof of that is him not getting or really even being seriously considered for jobs at Michigan, Tennessee, or Washington. Ferentz is an average recruiter, a decent Xs and Os coach, and a guy who can make a lot more money coaching in the pros than he ever would at Iowa. He's also been accused of being pretty shady something that on the NCAA level is not going to win you favors. That's not the case for Pete Carroll or Bob Stoops among others. Stoops made over 6 million dollars this year and he'll make a million more if the Sooners win the National title. He could very well jump to the NFL (rumored as being on the Denver Broncos shortlist) and the only reason he'd go is to prove he can do it, for self gratification.
Nick Saban thought this was a good idea when he left LSU and then realized what a mistake this was. In the NFL you manage much bigger egos, you have much less control, and you work a lot more hours during the season. I think Stoops would be nuts to leave Oklahoma where is is a God and rarely questioned but some coaches have to scratch that itch. But Ferentz isn't Stoops, he's a slightly above average coach who isn't getting long looks for the best jobs.
But yeah, it's ego. I can't blame guys with tons of success at the college level for wanting to give the NFL a go but to me college jobs are better because of the control and because of the stress. Although as a college coach there are more politics (recruiters, administrators, faculty, etc) you still don't have to deal with some headaches that are particular to the NFL like say Plaxico Burress shooting himself in the leg.