Here's why:

* Convergent Plate Boundaries: These are areas where tectonic plates collide.

* Continental-Continental Collision: When two continental plates collide, they push against each other with immense force. This causes the land to buckle, fold, and uplift, forming mountain ranges. The Himalayas are a prime example, formed by the collision of the Indian and Eurasian plates.

* Oceanic-Continental Collision: When an oceanic plate collides with a continental plate, the denser oceanic plate subducts (slides) beneath the continental plate. This process creates volcanic activity and uplift along the continental margin, forming mountain ranges. The Andes Mountains in South America are a good example.

Other Factors:

While convergent plate movement is the primary cause of mountain formation, other factors can contribute:

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

* Erosion: Weathering and erosion can carve out mountains, creating distinctive shapes and features.

* Isostatic Adjustment: The Earth's crust is in a state of equilibrium. As mountains are uplifted, the underlying mantle flows outwards, causing further uplift.

Let me know if you'd like more detail on any of these processes!