This is lazy writing. The Warren Commission’s initial investigation of the JFK assasination was a coverup of CIA anti-Castro activities and probably Oswald’s defection to Russia. And a real congressional investigation — not a circus like the Benghazi hearings — found that they could not rule out a conspiracy. Area 51 is, in fact, a top secret Air Force research and development facility that flies weird looking experimental aircraft at odd hours, with the Air Force quite happy to let people believe they’re UFOs. And Alger Hiss? It pains me to say this, but he might actually have been guilty, just as it turns out that Julius and Ethel Rosenberg were likely Russian spies.
There certainly are baseless conspiracy theories out there, but the examples picked aren’t exactly that.