Generalization such as this is no different than ASSUME.......
While it is true in many areas, it is not true in all areas - and most especially not in states where the public school teachers are not union. My daughter just started 9th grade, but is taking geometry because she took algebra in 8th grade - her math teachers for both courses have Masters in Math. Her Honor's English teacher has degrees in English and her Science teacher degrees in Science.
I spent 12 years in Catholic schools in NYC and my daughter is getting just as good an education in public school in VA as I did, and far superior in some areas.
I believe that this thread was really about Union teachers and that those teachers who do not have an Education degree, but a degree in a subject (as your child's teachers do) is a much better way to go.
I live in a state (Washington) where the teachers are unionized. When I was looking at my eldest daughter starting school, I was appalled at the low scores that the children in my city were scoring. Reading at grade level was in the low 20 percentile, and math was around 19%. I had a very bright child who was already reading her books by age 4 (she was reading single words at 2). I felt it would be child abuse to put her into that school system. We did not have a lot of money; but, we took the hit so our children could get a decent education. When the educators are unionized, the entire system becomes about them, not about the children.
I'm happy that your children are able to get a good education from your area's public schools. But, I think, for the majority of people in the U.S., that is just not the case. Again, in the school district I'm in, they stopped the "Honors" program years ago after one of their "Honors" students showed up at a School Board meeting to explain that the school had simply not prepared her for college (and she was the Valedictorian of her class). She was an Indian-American and she was told that some students just aren't ready for the college experience. She was their top student, and she was told that obviously she was not ready. It was such b.s. Anyway, after that incident they ended up having to admit that their Honor Program was just like their regular program; but, the students were the bright students who didn't ever disrupt the class.
So, I am glad I did the Catholic school route, and my kids got a decent education. Our finances would not have taken such a hit if I felt that my kids would have been served well by the public schools; but, I just wasn't ready to take the chance after what I had been reading and observing about the schools in this area.