I offer my take at the bottom of this Bloomberg column, but here are some reasons for the ones I reject:
To approach such an investigation, you might ask how much you believe improbable testimonies from witnesses who give every indication of being normal people. If you find sane witnesses persuasive, you might think there is some chance of UFO accounts being true (perhaps with a conspiracy-based coverup). There have been a variety of sober accounts of UFO visitations, most notably the story of Betty and Barney Hill.
Unfortunately for this nomination, psychological research on self-deception and the literature on the unreliability of witness testimony suggest that our minds can talk us into believing all sorts of things happened that actually didn’t. So witnesses don’t sway me much. I notice also that UFO claims have plummeted since the advent of mobile phones with cameras (“Oh, did you get a photo of them?”). And so I must look elsewhere for the most plausible conspiracy theory. The Bigfoot and Yeti tales take a tumble for similar reasons, and I don’t think anyone actually saw Elvis or Jim Morrison walking around in the 1990s.
Another way to search for true conspiracies is to scour history for deathbed confessions. Did any Cuban or Soviet agents, shortly before dying, blurt out that they knew the true story of President John Kennedy’s assassination? As far as I know, these admissions are hard to come by. That’s another reason for not believing in most conspiracy theories.
There is much more at the link., including on Malaysian Airlines Flight 370 and the Trilateral Commission, not to mention sports betting.