It's a matter of math. The average public company CEO has 7-10 C level reports. When that CEO leaves, they can't all get promoted.
I mean it happens at all levels, it's just more news at the C level. If you want to be the VP of sales for example, and you're currently a regional director, there are probably a few other regional directors who want to replace your VP. Not everyone can do it.
I have been thinking of "Yishan-style CEOs" for a while now, and part of point is to deal with things like this better (including board of directors doing CEO searches publicly). Anil Dash once had a joke about it: https://twitter.com/anildash/status/893153626247626752
I mean it happens at all levels, it's just more news at the C level. If you want to be the VP of sales for example, and you're currently a regional director, there are probably a few other regional directors who want to replace your VP. Not everyone can do it.