A lot of times those are players who hang around and who want to coach.
But think about all the nerds out there who weren’t big enough to play. Look at guys like Mike McDaniel. Let’s give those guys a laboratory to learn the game. Let’s teach them schemes and philosophies. Those guys aren’t going to get jobs as grad assistants because they’re small little guys. Why waste years learning how to teach a DB to backpedal or a WR to run routes when they could be studying the game at a much deeper level.
Coaches are also dependent on the coaches above them and whether they are successful or not. That’s also not great.