Here's why:
* No Legendary Encounters: The Desert Underpass is a regular route in the game and doesn't have any legendary Pokemon encounters.
* Specific Locations: Legendary Pokemon in Pokemon Emerald are found in specific locations:
* Groudon: You need to complete the game first and then find Groudon in the Cave of Origin (a new location that opens up after the game is beaten).
* Kyogre: Similar to Groudon, you need to find Kyogre in the Cave of Origin after completing the game.
* Rayquaza: You can catch Rayquaza after defeating Groudon and Kyogre, in the Sky Pillar, a location that becomes accessible after the main story.
* Other Legendaries: Other legendary Pokemon, like Latias and Latios, are also found in specific areas.
To get the legendaries in Pokemon Emerald, you need to follow these steps:
1. Complete the game: Beat the Elite Four and become the Pokemon League champion.
2. Access the Cave of Origin: This location is unlocked after completing the game.
3. Choose Groudon or Kyogre: You can only get one of these legendaries. Choose the one you prefer.
4. Find Rayquaza: After defeating the chosen legendary, go to the Sky Pillar and catch Rayquaza.
Remember: You can't get all of the legendaries in one playthrough. You will need to complete the game multiple times, each time choosing a different legendary at the Cave of Origin.