Probably because they never heard of it. And plenty of uninformed people (specially a couple of decades ago) think a "pre-professional’ education is the only acceptable path.
Plenty of US kids, including from private schools, head to Canada. Plenty head choose in state publics.
Kids who have intellectual curiosity and good work ethics can make the most of whatever college they end up at. If you don’t have that, Harvard is not going to save you. Some people believe a private school is more likely to foster that type of growth.