Here's why:

* Intrusive igneous rocks: These form when magma (molten rock) cools and solidifies *inside* the Earth's crust. They are often found in underground formations called plutons.

* Extrusive igneous rocks: These form when lava (magma that reaches the Earth's surface) cools and solidifies *on* the surface. This can happen in volcanoes, fissures, or other openings in the crust.

While extrusive igneous rocks form *above* the Earth's surface, the process that creates the magma itself happens *below* the surface. So, both types of igneous rocks ultimately originate from the cooling and solidification of molten rock, which originates within the Earth.