Here's why:

* Cooling Rate: Extrusive rocks cool quickly on the Earth's surface, often in contact with air or water. This rapid cooling doesn't give the minerals enough time to form large, visible crystals.

* Glassy Textures: If cooling is extremely fast, the molten rock may solidify before crystals can form. This results in a glassy texture, where the minerals are amorphous (lacking a crystalline structure).

* Fine-Grained Textures: Some extrusive rocks do contain crystals, but they are very small and require a microscope to see. This is known as a fine-grained texture.

Examples:

* Obsidian: A volcanic glass with a glassy texture.

* Pumice: A volcanic rock with a glassy texture and many air bubbles.

* Basalt: A fine-grained extrusive rock with microscopic crystals.

So, while some extrusive rocks have crystals, others can be entirely glassy or have very fine-grained textures due to their rapid cooling.