Thank you sir. You put words to my theorizations. It takes a lot of sacrifice to be able to simply claim that you are a college football coach. I believe, if it was myself launching a career, I would find it more rewarding, albeit potentially less glamorous, to spend my time with high school kids.
Most college coaching, like a lot of avenues in life, is simply who you know. That's why bad coaches keep popping up, because they always have a friend who'll give them a spot. Usually you make these in-roads as a player and parlay those into a GA job after you graduate. I think most HS coaches are simply guys who either didn't make those in-roads or specifically chose not to, they saw coaching as a calling but don't like the way it's done at the collegiate level (recruiting is not fun for a lot of people). The ones who move from HS to college are probably a mix of people seeking more of a challenge or simply those people who were students of the game but never played (this happens more than you think and usually these are the guys who do the really creative stuff because everyone else gets so mired in "tradition"). What bulldog said is also right, they pay the bottom guys almost nothing unless you're at an FBS job (some of the assistants on the teams in Last Chance U were living at those JUCO dorms, places barely bigger than jail cells), taking that kind of a pay cut is brutal and almost impossible unless you either live totally alone or your wife makes some good dough and can support you both while you chase down a dream.
Matter of fact, that first sentence goes for college and professional coaching is just about any sport. Getting your foot in the door is the hardest part but once you're in it's very difficult to get tossed out on your rear. Doubly so in the pros, it's always the same 30-40 guys just rotating around every few years.