You're right. It's also something employers can't dress up often for legal reasons. E.g. you can't be vague/fluffy/nice about the reason for putting someone onto a PIP. So if your company culture is kind/gentle, then it can seem like a HUGE abrasive shock to have one appear.

There are of course some people who are a) actively assholes and b) genuinely oblivious to the rules applying to them. But you gotta believe they're in the tiny minority in the world.