But how many high school calculus teachers spend their evenings doing calculus? (What would that even mean?) Can you imagine a geometry teacher spending his evenings writing faux-formal proofs with modus ponens? Do algebra teachers even use algebra? Do they depend on it?
Can you trust a teacher who doesn’t use what he teaches? Who has never used what he teaches?
Can you trust a teacher whose only connection to a subject is teaching it?
How can such a teacher know if what he’s teaching is valuable, or how well he’s teaching it? (“Curricula” and “exams”, respectively, are horrendous answers to those questions.)
Real teaching is not about transferring “the material”, as if knowledge were some sort of mass-produced commodity that ships from Amazon. Real teaching is about conveying a way of thinking. How can a teacher convey a way of thinking when he doesn’t genuinely think that way?
I’m sure many teachers spend their evenings thinking about teaching the subject. I have no doubt that these teachers love teaching, and love their students. But to me, that seems like a chef who loves cooking, but doesn’t love food. Who has never tasted his own food. This chef might have the best of intentions, but someone in need of a satisfying meal is probably better off elsewhere.