Here's why:
* Different fishing mechanics: Pokémon Ruby and Emerald use different fishing mechanics. Ruby uses the "Rod" item, while Emerald uses the "Old Rod," "Good Rod," and "Super Rod" to catch different types of Pokémon.
* Game-specific items: The developers designed the Old Rod specifically for Emerald to introduce new fishing possibilities.
What you can do in Pokémon Ruby:
* Use the Rod: This is the standard fishing rod available in Ruby. You can purchase it from the Department Store in Slateport City after obtaining the first Gym badge.
* Catch different Pokémon: While you won't find the same variety of Pokémon as in Emerald, you can still catch a decent selection of Water-type Pokémon using the Rod.
If you're looking for the Old Rod, you'll need to play Pokémon Emerald or FireRed.