PMs are responsible for keeping projects on schedule. Engineers are responsible for completing work.

Consequently, PMs are incentivized to compresses schedules, and engineers are pressured to do the same.

The end result is that "the people who do the work plan the work" goes out the window, because risks aren't fundamentally understood (on a technical nuance level) by PMs, so naturally their desire for schedule wins out whenever there's a conflict.

(That said, I've worked at shops that hew closer to how it it should be, and it works great. Current job just happens to be a dumpster fire of bad Agile)