Here's why:

* Formation: Gypsum forms through the evaporation of water containing dissolved calcium sulfate. This process typically occurs in shallow seas, lakes, or lagoons. The dissolved minerals precipitate out as the water evaporates, forming layers of gypsum.

* Igneous and Metamorphic: Igneous rocks form from the cooling and solidification of magma or lava. Metamorphic rocks form when existing rocks are transformed by heat, pressure, or chemical reactions. Gypsum doesn't form through these processes.

So, while gypsum can be found in association with igneous or metamorphic rocks, it itself is formed through sedimentary processes.