Why can conspiracy theorists never be pacified?

No matter what sort of evidence is released regarding any controversial event in history, it seems most conspiracy theorists will go out of their way and make outrageous and far fetched claims to keep the conspiracy going. I spoke to one today who said even if they could touch the body of Osama Bin Laden, they would believe it was a fake and that the government had set them up. Do they live to doubt?