Conspiracies do exist but are almost invariably unmasked. Conspiracy theories, which are by all rational measures unsustainable, persist because people delude themselves into only accepting evidence for the conspiracy and conveniently ignoring the evidence against.
The principle of Occam's razor suggests that the simplest hypothesis is usually the correct one — or as the character Gil Grissom in "CSI: Crime Scene Investigation" succinctly puts it, if you hear hoofbeats, "think horses, not zebras."
(VOODOO HISTORIES The Role of the Conspiracy Theory in Shaping Modern History. By David Aaronovitch)