Here's why:
* Jirachi is not in the game's code: Jirachi was introduced in Pokemon Ruby and Sapphire, which came out *after* Pokemon Emerald.
* No cheats can add Jirachi: While some cheat codes can manipulate certain aspects of Pokemon games, adding a Pokemon that's not programmed into the game is beyond their capabilities.
How to get Jirachi:
* Event distribution: In the past, Jirachi was given out at special events, usually through a special code distributed by Nintendo. Unfortunately, these events are long over.
* Trading: You can potentially trade with someone who has a Jirachi from a later game. However, this requires finding someone with a copy of a newer game and a Jirachi.
Let me know if you have any other Pokemon questions!