Here's why:

* Oceanic crust is constantly being created and destroyed at plate boundaries. New oceanic crust is formed at mid-ocean ridges where magma rises from the mantle. This new crust pushes older oceanic crust away from the ridge, eventually sinking back into the mantle at subduction zones.

* Continental crust is much more stable and less prone to recycling. While continental crust can be deformed and uplifted, it's generally not subducted back into the mantle. This means it can persist for billions of years.

Therefore, the oldest rocks on Earth are found on the continents, with some dating back over 4 billion years. Oceanic crust, on the other hand, is typically less than 200 million years old.