Why do you generally only find fossils in sedimentary rocks, and not in igneous or metamorphic rocks?

Fossils are organic products which are preserved in the Earth's crust. This means that the fossil is usually formed on or under (but not too deep!) the Earth's surface or under the water ... That's why fossils can't be found in the igneous rocks. The metamorphic rocks also can't contain any fossil as you'll see below.