Here's why:
* Arceus is a Mythical Pokémon: It's not found in the wild, and its availability is generally limited to specific events or distributions.
* HeartGold's Release: Pokémon HeartGold was released in 2010. Arceus was not available in a way that could be accessed with codes at that time.
How to get Arceus in other Pokémon Games:
* Event Distribution: Arceus has been distributed through events in various Pokémon games, often as a limited-time offer. Check online resources like Serebii.net for information on past events.
* Trading: You may be able to obtain Arceus through trading with other players who have it.
* Game-Specific Methods: In some games, you may be able to obtain Arceus through specific methods like completing a certain quest or fulfilling a specific set of requirements.
Remember: Using cheats or codes can potentially damage your game save file or lead to other issues. Always be cautious and research thoroughly before using any external tools.