Some of the methods these teachers use are incredibly awkward, bespoke approaches that baffle me for their obtuseness. It's incredibly frustrating to deal with and it has a negative effect on the students.
My approach has always been to try to find the method that makes the most sense to the student, and work with them on that. I don't have any issues adapting my style to the student's needs. I only struggle when the spectre of one of those bad math teachers looms over our shoulders.
Someone would have to be numerically illiterate (with a college degree!) to be unable to do grade school math they've been teaching for multiple years.
Year 1, possible. But eventually it rubs off on even the teacher.
An interesting thing about 3rd grade math - I was taught long division. When implementing the standard C library for the PC, I implemented the same algorithm in assembler, and then again for the floating point divide emulator.
Most people suck at math. Those who don't suck usually have many well-paying jobs available for them. There are lots of schools, so many math teachers are needed.
Put these numbers together, and you realize that there is no way to have math teachers who are actually good at math. The numbers just don't add up.
I would disagree with this. Those are necessary but not sufficient. It is necessary to have enough knowledge and joy from the subject to convey that to students.