When I was in school for my master's degree some years ago, several of my classes were heavily populated by teachers (New York State requires teachers to have or get a master's degree within 5 years of being certified). All were humanities teachers (English, Social Studies, ect - no STEM). At least half of them had great difficulty simply writing a one page essay. With one or two exceptions, reading comprehension was absolutely abysmal. At least two of them were functionally illiterate (in a master's program!). All were certified teachers who were actively working in schools.

The fact is that in many places school standards have been so low and social promotion has been going on for so long that we now have people coming out of high school and college that have never achieved anything academically. Many of these people go into teaching (even when schools were academically rigorous, majoring in education was always regarded as one of the least challenging areas of study).

That isn't to say that there aren't good teachers, or that there aren't smart teachers - there certainly are. It is to say that having an education degree or a teacher's certificate does not mean that one is qualified to do anything.

Does this mean every parent is smart enough or cut out to properly home school their child? Of course not! What it means is that (many) schools have effectively failed as institutions and until they are improved many people are going to look for alternatives.