I've also worked with a lot of awful product managers. The product manager title is squishy enough that it gets assigned to people with charisma or confidence without actual skills to follow through. A bad product manager can blend in to a company for years by relaying ideas around from one group to another and having ChatGPT write documents. The engineers on the ground see the incompetence long before it becomes undeniable at the higher ranks.
When I read Hacker News and other sites I suspect a lot of engineers have only ever worked with bad PMs from the latter category.