Why are rocks on the moon older than any rocks on earth?

On Earth, plate tectonics and erosion have removed all of the rock that was originally surface rock, exposing the underlying crustal rocks and releasing magma to form new igneous formations. On the Moon, neither process is at work. Meteoroids striking the Moon have piled the rocks up and created a layer of fine powder there, but the exterior rocks are still those that formed shortly after the Moon itself.