Here's why:
* Ditto's ability is Transform: Ditto's primary ability is to transform into the exact form of the Pokémon it's battling against. This is a temporary transformation, and it reverts back to its normal pink blob form when the battle ends or if it faints.
* No permanent transformations: There is no way to permanently change Ditto's form in the Pokémon games.
So, even though Ditto can transform into Lucario, it will always return to its original form.