Here's why:
* Fossil revival is a feature introduced in later generations. Pokemon Emerald is the third generation, and the ability to revive fossils doesn't appear until Pokemon Diamond and Pearl (fourth generation).
* Fossil Pokemon in Emerald are obtained through trade. Instead of reviving them, you'll need to trade with someone who already has a Fossil Pokemon.
If you're looking to revive fossils, you'll need to play a Pokemon game from generation IV or later.