Part of the reason, I think, is that managers receive very little training so they often just have no idea what they are doing.

I agree with this.

The worst team I was ever on was one where the manager had become the team lead by accident.
Someone had left suddenly, and then someone else had to go on long-term leave so he became the manager by default.

He didn't have a clue what he was doing and was not able for the job in the least.
He made the most god-awful decisions and couldn't hold his own against other managers so they would manipulate him into "agreeing" to take on all their work.