Here's why:

* Oceanic crust is denser and thinner than continental crust. It's primarily composed of basalt and gabbro.

* Continental crust is less dense and thicker, made up of a variety of rocks like granite and gneiss.

* The lithosphere is the rigid outermost layer of the Earth, encompassing both the crust and the uppermost part of the mantle.

* Since oceanic crust is thinner, the lithosphere beneath it is also thinner. This is why the lithosphere under oceans is more easily subducted (pushed down) at plate boundaries.

Let me know if you have any other questions about the Earth's structure!