Here's why:
* Game mechanics: Zen Mode is a special ability triggered during battles. It's a part of the game's mechanics, not a permanent transformation.
* Narrative purpose: Darmanitan's Zen Mode is a way to represent its fiery nature and the power it unleashes in combat. It adds a dramatic element to its battles.
* Balance: If Zen Mode was always active, Darmanitan would be incredibly powerful all the time, making it overpowered compared to other Pokémon.
Essentially, Zen Mode is a temporary boost Darmanitan gets during battles, allowing it to unleash its true potential.