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In my experience there are quite a few tenured professors that are brilliant in their respective fields (so i heard), but we're absolutely terrible in teaching their it. In my case this was physics (and also mathematics where i met some of these specimens). I suspect if you understand a certain field so naturally and really excel at that it becomes a second nature it it is more and more difficult to put yourself in an outsider's perspective. It is so foreign and unimaginable for you that someone might not understand this and that aspect naturally that you cease to be a good teacher in this.
In my experience, half of my engineering professors were terrible teachers
Understanding the subject is necessary but not sufficient to being a good teacher.
This phenomenon is known as the "curse of knowledge".