not everyone agrees with your premise.
First, not everyone agrees that a good public school education is as good as one at Prep or JA. I'm sure there are those that think it is somewhat better, and are willing to pay for any improvement, seeking the best possible education for their child, price be damned. And whether you agree or not, there are those that believe the education is vastly superior at one of the better private schools. Just like the average ACT isn't a good measure because of adverse selection in public schools, the fact that the Valedictorian at NWR got a free ride to Yale (need based only unless things are different nowadays) isn't statistically significant either.
I will tell you this: I know someone very well that has his kids at what most everybody (myself included) would call one of the state's best public schools. His kids are smart, and they NEVER bring home homework or study at home. And I do mean never. And they make all A's. Prep is not teaching at some astronomical intelligence level. They are justintent on instilling academic discipline. You will have homework and you will have to study. And if you fail to do those, you will get pounded grade-wise -- mostly on the homework end. I happen to believe that on the whole, that prepares kids well for higher education and for life. Does it work every time? Hell, no. But, statistically, it does.
So, at the very least, there are people who don't agree with your premise. And they are willing to pay for what you don't believe exists there. Could be that you are wrong.