Consider that lots of people regularly and voluntarily give a certain percentage of their income to their religious institution, without any kind of enforcement structure. There is a belief that such practices are good, right, and required. And it must be a strong belief, because people do love their money.

Second, taxes are not a gift or donation, nor is the government just taking them from you. They are payment for services. It's true that you are locked into a business transaction with the government, but it's meant to be a fair exchange. And unless you are incredibly wealthy, you have a favorable rate in the sense that the services you receive are worth more than what you're paying for.