Here's why:

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which are fragments of pre-existing rocks, minerals, or organic matter. Shale is formed from the compaction and cementation of very fine-grained sediments like clay and silt.

* Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava).

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions.

While shale can be transformed into metamorphic rocks like slate or schist under intense heat and pressure, shale itself is a sedimentary rock.