Why do monks shave their heads?

When Manchu people entered the customs, they asked the Han people to remove the hair from their foreheads and braid it, and ordered them to "keep the hair, keep the hair." As a result, it was strongly resisted by the Han people. Of course, the Han people died miserably. Monks have no hair, so naturally they don't shave their heads. Obviously, the Buddha had foresight. It also shows that it is correct to do more than one thing.