Maybe because there are so many incompetent PMs? The PM working I'm working with does not know the product feature set (except for the very basics), does not have any vision or goals, does not have a systematic approach to talking to collecting feedback from customers. What's on our backlog changes dramatically and when project ends we basically start with a new product backlog. When customers have questions on basic stuff he has to defer to engineering.

Any competent hire can be useful, but hiring good PMs is maybe even harder than good engineers. So people bitch.