This is partly a side effect of the, at this point, long-running trend of employers not training people on anything, let alone proper management.

I've had some awful managers in my career. One was formerly an elementary school teacher. She, very predictably, treated everyone like they were in elementary school, going so far as to give people gold stars for things. When called out on it, she confessed she really had no idea how to manage adults.

I've only had two excellent managers in my career, people who had studied management, were also technical, and cared about being effective at their jobs so the people they managed could be effective as well. These folks had formal training, undertaken of their own initiative, and it absolutely showed in all facets of what they did. Top notch folks!

I suspect this lack of training is an American phenomenon, but have no data to back up the claim.