People love conspiracy theories. It's far more exciting to believe that Lee Harvey Oswald worked for the CIA, or the Russians, or the Cubans, or Jimmy Hoffa, or Lyndon Johnson, or J. Edgar Hoover, or the Mafia, than it is to believe that a social misfit with a gun was able to kill a president and change history. That plus it's a little worrisome that our security net is so tenuous that a 24-year-old of moderate intelligence can figure out a way to get past it, and one wonders how many other social misfits there are out there just waiting for their opportunity. So I think that evidence or no evidence, a lot of people will always choose to believe that things happen because of diabolical plots by powerful forces rather than that a social misfit with a gun acted alone.
I believe in small-c conspiracies in the sense that people with wealth, power and influence will do what they can to hold onto them, which sometimes includes joining forces with other people of wealth, power and influence. That is not, however, quite the same thing as believing that the Pentagon invented AIDS in a laboratory, or that 9/11 was an inside job, or that Osama bin Laden was on the CIA payroll, all of which I've heard from one or another unnamed reliable source.
My skepticism is based on two factors which seem to me to be powerful evidence against such conspiracies. First is the well-documented incompetence of the type of people most likely to try to pull such conspiracies off. Think Bay of Pigs. Think Jimmy Carter's attempt to rescue the Iran hostages. Think Iran-Contra. Think of how many times we shot at bin Laden and missed before we finally got him. Think "mission accomplished". Think Operation Fast and Furious. Think of the Obamacare Website rollout. Think of the NSA's inability to detect and stop Chelsea Manning and Edward Snowden. For that matter, think of the war on drugs. It may be that at the top levels of our government there exist people with the competence to pull off 9/11 as an inside job and get away with it, but that's not the way to bet. This is not to say that there aren't evil people who attempt evil acts; just that a conspiracy is far more likely to look like fast and furious than it is to look like a model of efficiency.
The second reason for my skepticism is that I know how much trouble it is to keep my own birthday a secret. There are some kinds of secrets that just don't keep well, and most of these conspiracy theories are of that kind. Spend a few minutes actually thinking through, if Lee Harvey Oswald was part of a grand plot, how many people would have had to be in on it. And nobody has blabbed? Everyone has managed to keep quiet about it? Sure, I'll believe that. Just as soon as I start believing that George W. Bush was well-read on the history of the Middle East before he decided to invade Iraq.
It may be exciting to believe that we are surrounded by all of these dark and sinister plots. Show me some evidence. In the meantime, Occam's Razor is still in full force and effect.