Here's why:

* Formation: Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to intense heat and pressure deep within the Earth's crust. These conditions often exist over vast areas, especially near tectonic plate boundaries where mountains form.

* Mountain Ranges: Large mountain ranges like the Himalayas, the Alps, and the Andes are prime examples of regions where metamorphic rocks are found extensively. The intense pressure and heat during mountain building create these rocks over vast areas.

* Regional Metamorphism: The process of regional metamorphism, which occurs over large areas due to tectonic forces, is the most common way metamorphic rocks form.

While small patches of metamorphic rock can occur due to local events like igneous intrusions (where magma heats surrounding rocks), these are generally smaller in scale compared to the vast areas covered by regionally metamorphosed rocks.