There are only thirty-two teams. If you got fired, there's only so many options to do with your skills, take on another head coaching job, go down to offensive head or defensive head coaching job, go down to college coaching or become a TV color commentary analyst. That means there are only 32 head coaching jobs. If you want to move up into that position, someone has to quit, get fired, or die. NFL head coaching jobs pays pretty good. Even for the worst NFL team. It's a chance to turn a team around, even the worst team.
That's what coaching is about. You take it as a challenge. This is the same on the college level. You work with what you have. Player/students are always graduating and enrolling in the school. No two years are alike. On the pro level, your players get injured or get traded. You work with what you have.
These head coaches live a life of luxury. It's a high stress job but they live in the "nice" part of town in a big house, just like the top franchise players. No one does MTV Cribs at a coaches house, but they are living large. Even Div I college coaches of a major program get cash money.