Look, I'm trying to describe reality; you seem to be expecting me to defend it. But briefly:
> Inevitably because why?
Because proven, effective managers are always in short supply, so when you hire new people, or if any of the existing managers leaves, it's the default pick.
Plus, most people want to make more money over time. And on the management track, this means angling for that director / VP role down the line, even if it wasn't your childhood dream.
> If higher management can figure out not to put more people under them, why can't it figure out to remove the existing people under them?
They can, but in big and / or growing companies, performance problems are addressed less vigorously than they probably should. This cuts both ways: neglecting problems is wrong, but cutthroat performance management makes people cranky too.