Here's why:

* Conglomerates are formed by the cementation of rounded fragments of other rocks. These fragments, called clasts, are usually larger than 2 millimeters in diameter.

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which can be derived from pre-existing rocks (igneous, metamorphic, or even other sedimentary rocks).

* Igneous rocks form from the cooling and solidification of molten rock.

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

So, while conglomerates can contain fragments of igneous or metamorphic rocks, they themselves are classified as sedimentary due to their formation process.