Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock below the Earth's surface) or lava (molten rock erupted onto the Earth's surface). They are the original rocks from which all other rocks are derived.

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

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to intense heat and pressure, causing them to recrystallize and change their texture and mineral composition.

Therefore, even though sedimentary and metamorphic rocks are formed from other rocks, those other rocks ultimately originated from the cooling and solidification of magma or lava, making igneous rocks the fundamental source of all rock types on Earth.