For the same reason that the Earth doesn't need any fuel to keep orbiting the sun, and the moon doesn't need any fuel to keep orbiting the Earth..
Here's something to think about: A satellite in a perfectly circular orbit always stays at the same distance from the Earth, so it never moves in the direction of the gravitational force. The work that gravity does on it is (force) x (distance) = zero, so the satellite's energy never changes.
