Why? Every coach that has left under Harbaugh did so for a promotion and his job has been filled by someone with a resume just as good or better. Last year we lost our DC when he became the HC at Maryland. He was replaced by the DC of the #1 defense in the country. Our WR/QB coach just got the UCLA OC job. We just replaced him with the former Colts OC and Browns Assistant HC. I could go on and on but the point is coaches are lining up to work with Harbaugh because it is a fast track to the top of the profession.
Harbaugh is a crazy and weird mofo but he knows how to run a program/coach a team and other coaches know that (and want to work for him). I mean there are other coaches that have had to fire guys because they made bad hires...know anyone like that?