Spot on davejb
I used to think it was because the project managers did not know enough about engineering, but actually they are too close, and too involved in the engineering decisions, regardless of their past experience.
PMs should keep to their spreadsheets, and project plans, and leave the engineering decisions to the engineers. Put them in physically different locations to make sure they don't interfere.
Management by exception is the PMs mantra, but I see little evidence of it