They are too heavy, their arms are too small, and air is not dense enough.
The lift that flapping arms can provide is less than a kilo; not nearly enough to lift a person.
Much larger arms, such as the size of the Gossamer Condor, would be needed. The Gossamer Condor was the first man-powered aircraft.
If air were much denser, like water, arm flapping would produce more force. Swimming is like flying in water. We swim well because the flapping produces more force, and because the water provides a buoyant force that reduces our weight.
English lesson: a better wording is "Why can't people fly?" or "Why don't people fly?"
Alternative answer: Because people prefer the good food on airplanes.