First, not all of them are. Those that have left, I think, have done so for "career enhancing" promotions. Can't fault them for that.
Coach Kiffin said it a while back: It is a compliment when your assistant coaches keep getting good job offers elsewhere. It means you have good coaches. When did we ever hear of a significant amount of interest from other schools in our assistants under Fulmer (other than Cutcliffe)? That was not a coincidence.