Here's why:

* Continental crust is thicker and less dense than oceanic crust. It's made up of a variety of rock types, including granite and sedimentary rocks.

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

* Continental shelves are the submerged edges of continents, so they are still composed of continental crust.

Therefore, the crust under continental shelves is thicker than the crust under oceans.