Here's why:
* Giratina is not found in the Distortion World in the original games. It appears in the original Pokémon Diamond and Pearl games, but only after you have defeated Cynthia, the Champion, and obtained the National Pokédex. You can then access Giratina in the Turnback Cave, which is not in the Distortion World.
* The Distortion World is only accessible through the original Pokémon Platinum. This is a separate game and not a part of Diamond or Pearl.
If you're playing Pokémon Platinum, you can access the Distortion World through the following steps:
1. Complete the main story and defeat Cynthia.
2. Obtain the National Pokédex.
3. Go to the Spear Pillar. You'll need to have the "Azure Flute" item to access the Distortion World. This is received after speaking to a specific character at the top of the Spear Pillar.
4. Use the Azure Flute at the top of the Spear Pillar. This will open a portal to the Distortion World.
Once you're in the Distortion World, you'll need to find Giratina in a specific location. It's not just a random encounter.
Remember, the Distortion World is a unique feature of Pokémon Platinum, not Diamond or Pearl. You can't access Giratina through the Distortion World in the original games.