For some subjects it seems that the issue is not level, but quantity of work. They don't want to be seen as letting students get away with less work.
Maybe there's also something of a resentment about them not being needed when they encounter an autodidact? I had problems with an AP chemistry teacher who was angry that I was doing the homework in class instead of listening to the lecture. When I said that I thought textbook had explained the topic quite well, her reaction was extremely negative. The existence of the autodidact sort of threatens the proposition of school itself.
bruised egos are an understated problem when it comes to teaching. in my experience there are many teachers who react very poorly if you learn differently than their way.
one straight up refused to give me a syllabus after transferring schools to patch over the gaps in my knowledge because she 'planned it all in her head'
Maybe there's also something of a resentment about them not being needed when they encounter an autodidact? I had problems with an AP chemistry teacher who was angry that I was doing the homework in class instead of listening to the lecture. When I said that I thought textbook had explained the topic quite well, her reaction was extremely negative. The existence of the autodidact sort of threatens the proposition of school itself.