Here's why:
* No natural habitat: Celebi doesn't have a regular spawn location in the game.
* No in-game event: There were no events during the original release of HeartGold and SoulSilver that allowed players to obtain Celebi.
* Trading: Your only option to get Celebi in these games is to trade for it with someone who already has one.
If you're looking for Celebi, you'll need to try trading or look for other games where it's available.