Is it a bad thing? There are plenty of good public schools in the United States. They typically tend to geographically coincide with places with high average household income and thus lots of funding through property taxes, though that's not the only advantage money gives them.
> Why ought that to be the case?
Because society benefits when all children have at least some education from the age of five to sixteen, if not longer. There were times throughout history (up until relatively recently, actually - think prior to WWII) where there was lots of competition for your educational dollars. The problem was, if you didn't have educational dollars to spend, your kid got no education. And now that there's more urban than rural population in the US, you can't even just tell junior to help you with farm chores. You'll just have youths doing... well, it's hard to tell, but when kids don't have anywhere they have to be or anything they have to do, it usually turns out poorly for them and their communities.
> That's what I do, and the sticker price is less per year than the annual per-student opex of my local school district.
I bet it is. The average child who attends a private school typically comes from a household that has more financial resources, and thus, requires less of the social safety net that is often provided through school districts. These schools are also free to tell more "expensive" students (read: neurodivergent and disabled children) to pound sand when it comes to their needs.