Here's why:
* Evolution mechanics are different: In Pokemon Mystery Dungeon games, evolution is often tied to specific events or actions, like completing the story, achieving a high level, or even specific friendships.
* Focus on exploration and teamwork: The games focus on exploring dungeons, solving puzzles, and working with your partner Pokemon. They don't traditionally emphasize the typical Pokemon breeding and evolution mechanics.
So, if you need a Thunder Stone to evolve a specific Pokemon, it's not possible to get it in Pokemon Mystery Dungeon.