Here's why:

* Metamorphic rocks are formed from existing rocks (igneous, sedimentary, or even other metamorphic rocks) that are subjected to intense heat and pressure deep within the Earth's crust. This transformation process can take millions of years to complete.

* Igneous rocks form from the cooling and solidification of magma or lava, which can be relatively quick, especially for volcanic rocks.

* Sedimentary rocks form from the accumulation and cementation of sediments, which also takes time, but typically less than the process of metamorphism.

Therefore, the extreme conditions and the slow, gradual changes involved in metamorphism make it the rock type with the longest formation time.