If I had to guess, most of the original homeschoolers did it because they had strong, fringe religious beliefs. Then in your generations more secular parents started homeschooling but it was still a pretty highly self-selective group (a stay at home parent who didn’t mind staying at home most of the day teaching their kids, a belief they could do better at educating their kids even without the resources we have on the internet now to make it easier or the ability to use the internet to organize with other homeschooling parents, anecdotally a lot of either super-weirdo parents or hippy granola parents (more in unschooling)).

Now with the internet and WFH homeschooling is possible for many more parents than just those that don’t work fulltime and want to teach their kids full time, and it’s easier to create curricula, and easier to network and schedule things with other parents.

I do think there is probably a general problem of some pathologically bad parenting traits self-selecting much more than “normal” parents to homeschool: parents with control freak/micromanagement issues they subject their kids to, socially anxious parents who want to spend all their time at home with their family, parents with fringe/paranoid/extreme beliefs, having some kind of weird attachment or smothering issues they subject their kids to. Which isn’t a problem with homeschooling per se as much as it is selection bias in who chooses to homeschool.