There are actually a number of private Christian grammar, middle schools, as well as high schools. A simple search in a particular state or city will provide listings.
You’re not aware of it because you’re not a Christian and haven’t been closely acquainted in these circles, but these Christian institutions are not difficult to find.
In cities, yes. But many Americans live in small towns or suburbs where the only realistic alternative to public school is homeschool. My main point is that once you leave the big city, the number of private educational choices diminishes and I think that’s a pretty big deal.