Question

Why do you think that old rocks are rare on the ocean floor

Answer 1

At the mid-ocean ridges, lava is pushing the sea floor away from the ridge on each side. Eventually that part of the seafloor will be subducted under a continental plate at a convergent boundary. That means that the seafloor is constantly being renewed. The newest rocks will be found closest to the mid-ocean ridge and the oldest the greatest distance from the mid-ocean ridge. 
The oldest rock found beneath the ocean dates back only about 200 million years. 
However, the rock underlying the continents is as much as four billion years old.

Answer 2

It is believed that old rocks are rare on ocean floors because the floor has many remains and we are not unable to reach them.