Here's why:
* Skitty is not found in Pokemon Sapphire. Skitty was introduced in Generation III, but only in Pokemon Ruby and Emerald.
* Shiny Pokemon are rare in all games. While the odds are higher in some games, catching a Shiny Pokemon is always based on chance.
If you're looking for Shiny Pokemon in Sapphire, you'll need to find other Pokemon that are native to the game. You can check the Pokedex or online resources to find out which Pokemon are available in the game and where they can be found.