Here's why:
* Story-Driven Encounters: May is a main character in the story, and her role is focused on being your rival. You battle her a few times throughout the story, but after that, she moves on to her own journey.
* No Post-Game Encounters: Unlike some rivals in other Pokémon games, May does not have a post-game encounter where you can challenge her again.
If you'd like to battle a similar opponent:
* Challenge your rival Brendan: Brendan, May's rival, is also available for post-game battles in the Battle Frontier.
* Train and Battle Trainers: You can always challenge other trainers throughout the game, including gym leaders and Elite Four members, to hone your skills.
While you can't battle May again in Emerald, the game offers plenty of other challenges and opportunities to train your Pokémon and test your skills!