Why is that, because only a certified teacher can teach?
No,because curricula would suffer, amd the teachers simply would not be as good over the whole.
As it turns out, the average teacher actually is a better teacher than the average non teacher.
I don't know where you are getting your information from. Most people who home school have internet sites that help and provide information for teaching at home. What parents teach is no different than what school teaches perhaps outside of religion or BS classes like gym and home ed.
My tenants for example have two girls both home schooled. Once their work is complete, they are able to go outside and play. In school, they are basically baby sitting services and keep you there regardless how much school work you completed. There is no reward for studying.
I hated school when I was a kid. I think if we had home schooling back then, I would have been able to learn much more than in school. The mere threat of having to go to school if I didn't study at home would have been enough for me to bust my ass.