Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock) or lava. This is the fundamental process of rock formation.

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which are often derived from the weathering and erosion of existing igneous, sedimentary, or metamorphic rocks.

* Metamorphic rocks are formed when existing igneous, sedimentary, or even other metamorphic rocks are subjected to high heat and pressure, causing them to change in texture and mineral composition.

So, while the other types of rocks are created from existing rock, ultimately all rocks trace back to their origins in igneous rock formation.