Here's why:
* Venomoth's type: Venomoth is a Bug/Poison type Pokemon.
* Psychic type moves: Psychic type moves are primarily learned by Psychic-type Pokemon. While some other types can learn them, it's not common for Bug/Poison types.
* Move tutor: Venomoth can't learn Psychic from the Move Tutor in FireRed either.
If you're looking for a Psychic-type Pokemon in FireRed, you'll need to find or trade for one. Some options include:
* Abra: Evolves into Alakazam, a powerful Psychic-type.
* Drowzee: Evolves into Hypno, another Psychic-type.
* Kadabra: Evolves from Abra and is a very strong Psychic-type.