Here's why:

* Igneous rocks form from the cooling and solidification of magma (molten rock below the Earth's surface) or lava (molten rock erupted onto the Earth's surface).

* Sedimentary rocks form from the accumulation and cementation of sediments, which are fragments of other rocks, minerals, or organic matter. While sedimentary rocks cover a large portion of the Earth's surface, they represent a much smaller percentage of the total rock volume compared to igneous rocks.

* Metamorphic rocks form when existing igneous or sedimentary rocks are transformed by heat and pressure. While metamorphic rocks are significant, their overall volume is less than igneous rocks.

Therefore, even though sedimentary rocks are the most visible at the surface, igneous rocks make up the largest portion of the Earth's crust by volume.