Here's why:
* Generation Differences: Pokémon LeafGreen is a remake of the original Pokémon Red and Blue. These games were released before the introduction of the second generation Pokémon (including the starters you mentioned).
* Game Mechanics: The game code and mechanics of LeafGreen are designed to work with the original 151 Pokémon. There's no way to encounter or obtain the later generation Pokémon through normal gameplay.
Ways to Get These Pokémon:
* Trading: You can trade these Pokémon from a separate Pokémon Gold, Silver, or Crystal game using a compatible system and link cable.
* Game Sharer: If you're playing on a system with online functionality, you might be able to obtain these Pokémon through a game-sharing service.
* Game Hacks: There are ways to modify the game files to introduce new Pokémon, but this is generally not recommended as it can impact your gameplay experience.
Remember, it's important to play the game as intended and follow the rules of the original game if you want to experience the authentic Pokémon experience!