Here's why:

* Formation: Conglomerates form from the accumulation and cementation of rounded fragments of other rocks (gravel-sized or larger).

* Composition: These fragments, called clasts, are held together by a mineral cement, often calcite, silica, or iron oxide.

* Characteristics: The defining features of conglomerates are their large, rounded clasts, which distinguish them from other sedimentary rocks like sandstone (made of sand-sized particles).

So, while conglomerate is composed of pre-existing rock fragments, it's classified as a sedimentary rock due to its formation process and the way the fragments are bound together.