Here's why:

* Conglomerates are clastic sedimentary rocks. This means they are formed from fragments of other rocks (clasts) that have been cemented together.

* The clasts in conglomerates are gravel-sized, meaning they are larger than 2 millimeters in diameter.

* These gravel-sized clasts can come from various sources, including pre-existing sedimentary rocks, igneous rocks, or metamorphic rocks.

So, while conglomerates themselves are sedimentary rocks, the original source of the rock fragments that make them up can be from a variety of rock types.