Why can't you unlock all the treasure chests with a thieves key in Dragon Quest 8?

In Dragon Quest VIII, not all treasure chests can be unlocked with a Thief's Key due to certain plot points and gameplay mechanics. Some chests are specifically designed to remain locked until certain story events occur or require different keys later in the game. This adds an element of exploration and encourages players to revisit areas as they progress in the story. Additionally, it maintains a sense of challenge and reward throughout the adventure.