Here's why:

* Internal forces are forces that originate within the Earth. These include:

* Plate tectonics: The movement of Earth's tectonic plates causes collisions, which can push up land to form mountains.

* Volcanism: Volcanic eruptions can build up layers of lava and ash, creating mountains.

* External forces are forces that originate outside the Earth. These include:

* Weathering: The breakdown of rocks by wind, rain, and ice can wear down mountains over time.

* Erosion: The movement of water, wind, and ice can transport weathered rock, further shaping mountains.

While external forces can shape and modify mountains, they don't create them in the first place. That process is driven by the internal forces of the Earth.