Some people are fortunate to be in situations that enable them to stay in one place for a long time. Others are not, quite often for reasons out of their control. For instance, a head coach is fired, leaves for another job or retires and the new head coach does not retain the assistants. I've seen new ADs come in and force coaches out so that they can hire people more familiar to them. Happens all the time. Or family circumstances intervene. A parent falls into bad health, kids move away, or your spouse lands a great job which they just can't pass up. Programs get cut or resources get reduced. And unlike sports such as football and basketball, not that many cc/track coaches get buy outs or separation compensation beyond July 1. All of that makes it tough to blindly place your eggs all into the same basket for great stretches of time.
Here's hoping that all the good coaches who'd like to stay in their present jobs are able to do so.