FTA: "Kathryn Olmsted, a historian at the University of California, Davis, says that conspiracy theories wouldn’t exist in a world in which real conspiracies don’t exist. And those conspiracies — Watergate or the Iran-contra Affair — often involve manipulating and circumventing the democratic process."

So the answer to the question "Why Rational People Buy Into Conspiracy Theories" is "because sometimes they are true."

FTA: "if you think one of the theories above is plausible, you probably feel the same way about the others, even though they contradict one another. "

FTA: "“The best predictor of belief in a conspiracy theory is belief in other conspiracy theories,” says Viren Swami, a psychology professor who studies conspiracy belief at the University of Westminster in England."

Note how we suddenly switched from discussing people who think a theory is "plausible" to "belief". There are certainly nutjobs who swallow implausible theories whole. These people are not rational. Those who commit the crime of conspiracy would love to tar those rational people who find such theories plausible with the same brush.