I think this might be a post hoc explanation of an old fallacy. People become teachers, and are promoted to a visible level, based on a combination of teaching

I think this might be a post hoc explanation of an old fallacy. People become teachers, and are promoted to a visible level, based on a combination of teaching skill and expertise in the field. If someone was a bad teacher and not an expert, they'd be cut off. Thus, there are many teachers who are good at teaching with low expertise, and many teachers who are top experts with little teaching skill. I think there are also people who are top experts and good teachers (I've been fortunate enough to meet some), but they're obviously rare than the other two.