There's also the issue that in a home schooling environment, the student is vastly freer to just ask questions. In a school, there's a curriculum and a lesson plan to follow, so the only questions that get answered are along the lines of "how do I do (this thing you just asked me to do)".
Another way to put that is that public schooling has the same problem, and institutionally at least the solution is to just discourage those questions.
A parent teaching their child for months and years gets a pretty good idea of how their child learns. They don't need to know how to accommodate every learning style, they just need to figure out one.
On the flipside, I have had several real teachers about whom I frequently wondered why they had chosen teaching as a profession. It was like they lacked both the aptitude for it, and had an intense dislike of students. To be fair, most were not like this, and there were a few truly excellent ones. But to make the argument that they are all highly trained, excellent professions is a bit of a stretch.
That being said, you don't need to know all the material on day one. My mother did not know algebra when she taught it to my older brother the first time. She simply worked through the book a few lessons ahead of us, and if we ran into a problem we really couldn't answer, she reached out to friends and/or family who might be able to help. And I'm sure that wasn't the first or last time she encountered material that was new to her.
There are a significant number of resources and support organizations that can provide material, lesson plans, cooperative support, and many other things for those who aren't experts in a given field. I know many groups who collectively trade subjects leaning on friends who have a particular specialization.
If you teach your child how to learn on their own, they will be able to, at some point, take over their own education. That's what happen with myself and my two brothers. We all started taking primary responsibility for learning as we matured, and going into high school only occasionally leaned on our mom for assistance in a given subject.
The important thing here isn't that you as a parent know everything, but that you as a parent are willing to leverage the resources at your disposal and to invest time in learning when you need it.
There are so many things that happen when you are sitting in a class of 32 students that could be far better. Delays simply due to lagging students, teachers not having time to properly explain things, or get to understand and correct holes in their students' knowledge, tailor subjects to their tastes and interests, etc. Some random acquaintance who home schooled her children pointed out to us that their biggest problem - after finishing the government mandated curriculum for the day in 2 hours - was finding things to actually keep the kids busy for the rest of the day. It makes me wonder what an optimised education system could actually achieve.