Why are there ghost people at the old Chateau in Pokemon platinum?

You're likely referring to the "ghosts" you encounter in the Chateau, specifically in the room where you meet the Ghost type trainers. While they appear spectral and ghostly, they aren't actually ghosts.

Here's why they appear that way:

* Illusion: The trainers are actually under the influence of an illusion created by the "Ghostly Apparition" item held by the ghostly trainer, "The Phantom." This item causes the affected trainers to appear as spectral figures.

* Theme: The Chateau has a very spooky and mysterious theme. This illusion adds to the atmosphere and creates a sense of intrigue for the player.

* Story Element: The illusion is part of the storyline surrounding the Chateau and the Phantom. It sets the stage for the confrontation with the real trainer and provides a unique challenge for the player.

They are not actual ghosts, but rather living trainers disguised by a special item to create a spooky and memorable encounter.