Whereas this is something I AM trained to do in certain subjects but other people with no training at all seem to be happy to do in ALL subjects with no training whatsoever. It's more like saying "I'm a trained brain surgeon, but I wouldn't attempt heart surgery, and I'm a bit surprised that some people with no medical training think they are able to do diy surgery on any part of anyone's body".
But it's not really something you are trained to do because you are trained in one very particular way, one where you have to teach to pass particular exams, one where you have to control maybe 30+ kids at once. One where you get about an hour a day, 4 days a week to impart your knowledge in a very controlled environment, then gather yourself together move on to the next class of students learning at a different level and spend the next hour teaching them. It's really not at all comparable to facilitating your own child's learning.
I get why you feel it's undoable, I'm a history buff, a literary buff, I occasionally write professionally and have an excellent natural aptitude for maths and can do complex algebra problems in my head in seconds. But I don't speak any language other than English, apart from a bit of HTML I can't write code and while I devour science magazines I've never bothered to learn the full process behind the most of the studies and discoveries I know all about. My understanding is very superficial. So it's easy to assume that I would fail to give my son a well rounded education and would especially be unable to help him acquire a university place to study a STEM subject.
But I can inspire him in ways that no teacher in a classroom will ever be able to. He's only 4 now but we do basic physics and biology experiments all the time, like last year when he was hoping his new car seat would be forward facing so we spent a few days learning about momentum (which he knew about from Blaze) and the effects of rapid deceleration on the human body. And the different effects it has on a child due to their head being proportionally larger. We set up a wagon and got a selection of dolls and teddies and learned about the different outcomes based on no restraint, forward facing restraint and rear facing restraint. And different ways the toys reacted based on their proportional head size. By the end of it my 3 year old had a much better understanding of those concepts than I did when I left school.
Or when his camera stopped working last we worked out that the problem was with the power supply. Took it apart together, found the wires that were responsible for power and located the one that had disconnected from the motherboard, worked out where it should have been. Then I soldered it back in place while he watched from the other side of the table and I gave him a pretty full on description of the effects of molten metal if it gets on your hand. End result, he still hasn't worked out that he needs to hold his camera still for a second or two after pressing the button but he's learned about problem solving, motherboards and solder.
As he gets older he will have the tools to learn about whatever inspires him the most and I have the tools to show him how that subject is linked to others and pique his interest in ways that he can relate to. And as well as teaching him I'm insanely excited about all the things I too will learn on this journey. I'm not just his guide and facilitator, he is also mine because his needs and interests will inspire me to move out of my comfort zones. It's really just not at all like teaching in a school.