Why it is also not illegal for the government to give loans and grants for people to attend private colleges?
Isn't a college grant basically the same as a voucher?
Excellent point, but to understand why we need to understand the history and reason behind government education for K-12.
The “K” is for Kindergarten a German word literally meaning child garden. The Prussians were upset at their Napoleonic losses and blamed weak parenting. The state would raise up good, strong, brave Germans and start as early as possible to prevent parental corruption.
Americans inherited both the above and the British tracking system which the British saw employed in India. The tracking system paralleled the caste system. No need to teach an untouchable what a Brahmin needed to know.
American education was about as free market as ever with teachers paid by concerned and interested parents. With expanded Catholic immigration American Protestants feared Papists would take over the government and so wanted the education to indoctrinate Protestant values into the children of these new immigrants.
Toss in Social Darwinism and the Progressive movement and viola, you have our current witch’s brew of compulsory government education.