Here's why:
* Kinetic energy is the energy of motion. An object has kinetic energy if it is moving.
* A rock at the edge of a cliff is stationary. It's not moving, even though it has the potential to fall.
However, the rock does have potential energy. Potential energy is stored energy due to an object's position or state. The rock has potential energy because of its height above the ground. This potential energy will be converted to kinetic energy if the rock falls.
