Its an elite position, at the most elite level of that sport. And there are only 32 of them, if you're talking about head coaches. Take any other skill profession, for example. I'm a lawyer. There are nearly one and a half million of us. I was a criminal investigator before that. There are over 5 million of those.

There is turnover in the NFL and folks get canned for under-performance (just like any other job), its just a lot more visible to the public when management has to move on. Every coach you named was successful at the pro level as at least a coordinator. In a market that competitive, that doesnt always translate to being a good HC.