I once was in a sit down with Malzahn and we got into a pretty deep conversation. One thing that stood out is he said, "I think you are in your coaching prime between 30-40. In your 20's you have all the energy in the world but you lack the experience and intelligence. After 40, like I am, I don't have the energy that I used to. But in your 30's you are old enough to know what to do and still have the energy to do it."Chip Kelly is paid 40% as OC at OSU vs his salary as HC at UCLA.
For some it's not about the money - Gus has made many millions - but rather about what they want to do. I suspect many coaches prefer actually coaching rather than being a CEO, which is more in line with what a modern HC is.
This was 5 or so years ago so I can assure you he has lost even more "pizazz" since then. He very well could just want to take a step back and call plays without worrying about fundraising and all the thousands of other duties that a HC is expected to do.