Here's why:

* Fusion is a process, not a state of matter. Nuclear fusion is the process of combining atomic nuclei to release energy. It happens at extremely high temperatures and pressures.

* The reactants and products of fusion can be in different states of matter. The fuel for fusion (typically hydrogen isotopes) is usually in a plasma state (a superheated gas where electrons are stripped from atoms), but the products can be in different states depending on the specific reaction and conditions.

Think of it this way:

* Burning wood is a chemical process.

* The wood itself is solid, but the process of burning involves gases and produces ash (a solid).

* Just like burning wood, nuclear fusion is a process, not a state of matter, and involves various states of matter.