Here's why:
* Jirachi is an event-exclusive Pokémon: It was only available through special events, typically involving a bonus disc or code.
* Ruby has no event functionality: The game wasn't designed to receive event Pokémon like Jirachi.
To get Jirachi, you would need:
* A bonus disc: These were distributed at special events, allowing you to access Jirachi.
* A cheat device: You could potentially use a cheat device to modify the game data and add Jirachi to your game. However, this is considered cheating and can potentially damage your save file.
Therefore, the only legitimate way to obtain Jirachi in Pokemon Ruby is by using a bonus disc that was released for that specific game.