Here's why:

* Mountains form due to tectonic plate collisions. These collisions cause the crust to buckle and fold upwards, creating mountains.

* The crust is actually thicker under high mountains. The immense weight of the mountain range presses down on the underlying mantle, causing it to flow outwards. This process, called isostasy, creates a thicker crust beneath the mountains to balance the weight.

Think of it like a boat floating in water. The deeper the boat sinks into the water, the wider the base of the boat needs to be to support its weight. The same principle applies to mountains and the crust.