Here's why:

* Diamond and Graphite: The most common forms of carbon found naturally are diamond and graphite, both of which are solids.

* Other Forms: While carbon can exist in other forms, like fullerenes and nanotubes, these are still solids.

* Conditions: Carbon can exist in liquid form at extremely high temperatures and pressures, but these conditions are not found naturally on Earth.

Let me know if you have any other questions!