Here's why:
* Mew is not a naturally occurring Pokémon in the game. It was never intended to be encountered in the wild or through normal gameplay.
* There are no glitches or exploits to legitimately find Mew. While some glitches exist in older Pokémon games, none allow for catching Mew in HeartGold.
How people usually get Mew:
* Cheats: Using cheat devices or save file editors to insert Mew into the game.
* Event Distribution: In the past, some events allowed players to receive Mew via special distribution methods, but these are long gone.
If you're looking to catch Mew, you'll need to rely on methods outside the standard gameplay.