Here's why:

* Fine-grained igneous rocks form when magma cools and solidifies relatively quickly. This rapid cooling doesn't allow large crystals to form, resulting in a fine-grained texture that can be difficult to see with the naked eye. Examples include basalt and rhyolite.

* Glassy igneous rocks form when magma cools extremely quickly. This rapid cooling prevents crystals from forming altogether, resulting in a smooth, glassy texture. Examples include obsidian and pumice.

While metamorphic and sedimentary rocks can sometimes have fine-grained textures, these textures are generally not as common or as characteristic as they are in igneous rocks.