Here's why:
* Pikachu is a starter Pokemon: In Pokemon Mystery Dungeon games, your starter Pokemon is chosen at the beginning and remains the same throughout the game. It is a special case, and it's part of the storyline and gameplay mechanics.
* No evolution stones: The games don't use traditional evolution methods like level-up, stones, or trades. You don't have the ability to evolve any Pokemon through conventional means.
While you can't evolve your starter Pokemon, you can have other Pokemon on your team that might evolve. However, evolution in Pokemon Mystery Dungeon is usually determined by story progression, fulfilling certain requirements, or encountering specific events within the game.