Here's why:
* Ghost-type Pokémon are immune to normal and fighting-type moves. Fire-type moves are typically considered normal or fighting-type.
* Ghost-type Pokémon are weak to ghost and dark-type moves.
Therefore, if you're trying to battle a Ghost-type Pokémon, using fire-type moves will have absolutely no effect. You'll need to utilize a Ghost or Dark-type move to deal any damage.