Private schools arose from the apprenticeship tradition of training youth for specific trades or professions. From kindergarten through the 12th grade, the purpose is to get students into college or technical school; then is to get them into a trade or law school, CPA, MBA, medical school, etc.
This is done by teaching them when to think. The professional system is designed to create expertize. And if you need a doctor, a lawyer or a manager for your business, you are glad they are well prepared.