Here's why:
* Shiny Pokémon are rare: They appear randomly, with a 1/4096 chance (or about 0.024%) of encountering a Shiny Pokémon in the wild.
* Pichu is not a wild encounter: In Soul Silver, Pichu is only obtainable by breeding Pikachu. This means you can't encounter it in the wild and thus, you can't encounter a Shiny Pichu.
How to Increase your chances of finding Shiny Pichu:
1. Breeding: The most common way to get a Shiny Pichu is to breed two Pikachu together. You can increase your chances of getting a Shiny by using a Shiny Charm and/or the Masuda Method (breeding with a Pokémon from a different region).
2. Trading: You can find a Shiny Pichu through trades with other players. This is often the easiest way to obtain a Shiny Pichu if you're not into breeding.
Please Note: Finding Shiny Pokemon is purely based on chance. Even with the Masuda Method and a Shiny Charm, you still won't be guaranteed to find a Shiny Pichu. It just drastically increases your odds.