Other mistake: The actor playing John F. Kennedy in no way resembles the late former President.
Other mistake: When Dandy goes on his shooting spree, the pistol he uses holds 9 rounds. When Jimmy returns to find everyone dead inside the main tent, there are 11 dead bodies laid out.
Camp Redwood - S9-E1
Other mistake: Most of the mud on Brooke should have been washed off as she walked through the downpour to enter the cabin with the others.
Camp Redwood - S9-E1
Other mistake: When the group enters the infirmary, none of them are soaked from the downpour of rain happening outside.
Chosen answer: Because on Halloween, the souls of The Dead are said to roam the Earth. That includes those souls that are trapped within the walls of the "Murder House".
THGhost