Here's why:
* Storyline: The gym leaders are part of the main story and are defeated once you've earned the badges. The story is designed to progress you through the game, not to have you re-challenge them after the main story is complete.
* No Post-Game Content: Unlike some later Pokemon games, HeartGold and SoulSilver don't have a significant post-game content with rematches against gym leaders.
However, there are some ways to challenge yourself again in HeartGold and SoulSilver:
* Re-play the game: You can start a new game and challenge the gym leaders again.
* Trade Pokemon: Trade with friends who have different teams and challenge them in battles.
* Use the Battle Tower: This facility offers a challenging, competitive battle experience with a variety of trainers and difficulty levels.
* Use the Battle Frontier: This post-game area offers a variety of unique battle challenges and allows you to face different trainers with unique strategies.
While you can't specifically rematch the gym leaders, there are other ways to challenge yourself in HeartGold and SoulSilver!