"Those who can't do, teach" is almost exclusively about money, the idea being that in almost every industry you would be paid more to do than to teach. Thus, if you are teaching it's likely because you failed to be a doer. This isn't anti-intelligence, it's actually a fairly logical stance.
It does miss that a person's desire to educate can far exceed their desire to ply the trade and they are willing to sacrifice their pay at that particular altar. But given most teachers do not seem irrationally excited about teaching or being teachers that line of thought is quite diminished.
Being a teacher (as opposed to a trainer, like me) is really difficult, and a great many people who are excellent engineers and scientists, are horrendously bad teachers.
The best teachers that I ever had, were ones that were trained as teachers, and were not necessarily content matter experts.
The worst teachers that I ever had, were content matter experts. Almost universally, they had no patience for folks that had a hard time coming up to speed, or that weren't already at a level beyond the class they were teaching.
They would ridicule you for asking "stupid" questions (that's me -I ask questions that have the whole class in stitches, but by the end of the semester, I'm coaching my classmates). They would start from a baseline that actually assumed the student had already completed and passed the class they were taking.
I would sign up for a class, because of the bona fides of the teacher, but would end up regretting my decision.
It does miss that a person's desire to educate can far exceed their desire to ply the trade and they are willing to sacrifice their pay at that particular altar. But given most teachers do not seem irrationally excited about teaching or being teachers that line of thought is quite diminished.