Here's why:

* Oceanic crust is constantly recycled: The process of plate tectonics causes oceanic crust to be subducted (pushed) under continental crust. This process melts the oceanic crust, and it is essentially destroyed.

* Continental crust is more stable: Continental crust is less dense and therefore less likely to be subducted. This allows it to survive for longer periods and accumulate older rocks.

While some very old rocks have been found in the ocean, they are typically fragments of continental crust that have been eroded and transported to the seafloor. The oldest rocks on Earth, however, are found on land, specifically in places like Canada, Greenland, and Australia.