Here's why:

* Intrusive Igneous Rocks: These form when magma cools and solidifies beneath the Earth's surface. The cooling process can be slow, allowing for the formation of large crystals. However, these rocks are often massive and don't display distinct layers.

* Extrusive Igneous Rocks: These form when lava erupts on the surface and cools quickly. While some extrusive rocks can exhibit layering (e.g., in lava flows), this is not a defining characteristic of all igneous rocks.

In contrast, sedimentary rocks are almost always formed in layers. This is due to the deposition of sediments over time.