Here's why:

* Continental Crust: Continental crust is thicker than oceanic crust.

* Mountain Formation: Mountains form when tectonic plates collide, causing one plate to buckle and rise above the other. This process adds layers of rock, increasing the crust's thickness.

The Himalayas, for example, are a prime example of this. The collision of the Indian and Eurasian plates has created the world's highest mountain range and some of the thickest crust on Earth.