Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava). While some mica minerals can form within igneous rocks, they are not the primary source.

* Sedimentary rocks form from the accumulation and cementation of sediments. Mica can be found in sedimentary rocks as a result of weathering and erosion of metamorphic or igneous rocks.

* Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemically active fluids. Mica is a common mineral in metamorphic rocks, often formed during the recrystallization of existing minerals under these conditions.

In short: While mica can be found in all three types of rocks, its primary formation is in the metamorphic environment.