There was another case with a different science teacher where we were learning about weather in science class. The book showed a drawn pic of a rain gauge which had a funnel on top of it. Basically a glass cylinder with inches marked on it with a funnel on top to catch the rain. I asked the teacher: "Wouldn't that funnel mean it's catching more rain compared to the diameter of the cylinder and wouldn't that throw off the result making it look like there was more rain than there really was?" And he was adamant that with or without the funnel you'd get the same result.

Because they are incompetent and limited in their knowledge.

Just look at their salaries [oublic school system]. Their pay weeds out quite a lot of good people

American private school salaries are often lower than equivalent public school salaries, because you don't need credentials to teach at a private school, and because private schools can be an easier work environment than public schools.

I did three years in education before switching to software development. I would have done a lot more if the pay was better and the schools provided more support to the teaching staff. Sometimes I feel like I'm wasting my time working on a bunch of worthless marketing websites instead of educating people but at least I can provide for my family and live a rich and fulfilling life outside of work.

Better work environment, and private schools typically discount tuition significantly for the children of teachers. At current tuition rates, that's a massive benefit.