[ffsm8]

I think you're mistakenly equating mentoring with managing.

A manger inherently isn't your friend nor on your side, a mentor is.

It's straight up in the name: the goal of a manager is to remove your individuality from the perspective of the person above the manager. The manager is essentially an abstraction layer if you're looking at it from a software development perspective, so the caller doesn't need to worry about the implementation details.

Your manager can also be your mentor, but the manager can't mentor everyone they're managing. That's just impossible from a time perspective