Why do some people believe conspiracy theories? Because they would join conspiracies themselves.
British psychologists Karen Douglas and Robbie Sutton found that people with a machiavellian attitude toward deceiving and manipulating other people were more likely to believe in conspiracies to manipulate and deceive.
This makes me wonder what kind of people believe, as I do, that there are many "conspiracies" to help others and build up the world. Though I don't think of these as conspiracies so much as cooperation.