Here's why:

* Oceanic crust is constantly being created and destroyed at plate boundaries. At mid-ocean ridges, magma rises from the mantle, cools, and solidifies to form new oceanic crust. This process is called seafloor spreading.

* Continental crust is much older and more stable. It is less dense than oceanic crust and therefore does not subduct (sink) back into the mantle. This means that continental crust can accumulate over millions of years, becoming thicker and more complex.

* The oldest oceanic crust is about 200 million years old. This is because older oceanic crust is constantly being subducted and recycled.

* The oldest continental crust is about 4 billion years old. This crust has been preserved through various tectonic cycles and weathering processes.

In summary, the continuous creation and destruction cycle of oceanic crust leads to it being much younger than the stable and accumulated continental crust.