Here's why:
* Fly is a TM: Fly is a Technical Machine (TM) that can be learned by a wide range of Pokémon, but not Butterfree.
* Butterfree's Movepool: Butterfree's natural movepool in the games does not include Fly.
* Evolution: Butterfree evolves from Metapod, which evolves from Caterpie. Neither of these Pokemon can learn Fly either.
How to get around this:
* Use a different Flying Pokémon: There are many other Pokémon that can learn Fly, such as Pidgey, Pidgeotto, and Pidgeot.
* Use other Flying moves: Butterfree can learn moves like Gust, Air Slash, and Bug Buzz, which have similar effects to Fly.
It's important to remember that game mechanics can change between different Pokémon games, so it's best to check the specific game you are playing for move learning options.