Because the government wants (should want, its debatable rn) people to become educated and contribute to society, so they subsidize education. A lot of this money goes to research and scholarships, and while it's questionable why Harvard needs billions from the government when they already charge out the ass, it makes sense that the government (and in theory, the taxpayers too) want more people to get good education to help the country we all live in do better. This applies for a lot of colleges and universities, not just Harvard.
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