Here's why:
* Fly is a HM (Hidden Machine) in later Pokemon games. It's learned by a Pokemon and allows you to travel quickly between locations you've already visited.
* Pokemon Yellow focuses on the original 151 Pokemon. Fly and other HMs like Surf, Strength, and Cut are not part of this original generation of Pokemon games.
Instead of Fly, you'll have to rely on walking, running, and using the bike to get around in Pokemon Yellow. It's part of the classic Pokemon experience!