people are too self-interested and silly to ever carry out a conspiracy completely successfully.
But there's a problem with this argument: The set of known, successful conspiracies is a set of size zero. But that's because "known, successful conspiracy" is an oxymoron, rather than telling you anything about how many successful conspiracies really exist. If you uncover common traits of all known conspiracies, you are by definition excluding the successful ones.
As an analogy, imagine if you wanted to get tough on white-collar crime, so you studied every known case where someone got caught embezzling money, and then focused on tightening up the system for whatever those people did. The problem is one of perspective: you only looked at people who got caught, you're not looking for people who got away with it. So whatever traits were common to all those cases were in fact traits held in common by people who messed up.
Okay, to start off: Many conspiracies are created by charlatans, with the intent to exploit the gullible and paranoid. Discuss if you want.
The definition of "conspiracy theory" tends to be selective in terms of which conspiracies it includes, meaning there's a circular definition involved. e.g. is Watergate a conspiracy theory? There was a conspiracy, and "the truth" about it, is just a collectively accepted theory on what happened. So what does it mean if the only measurable difference between a "true conspiracy" and a "conspiracy theory" is that you've convinced enough people that the first one is true?
As for how charlatans fit into the picture, most fit into two common types:
- cult leaders, who use fear of outsiders/outsider-ideas to control their followers - religious, anti-government type conspiracies
- book writers / convention speakers - these people generally opportunistically graft onto some pre-existing myth, e.g. The Philadephia Experiment
But in general, most conspiracy theories are not created by people for profit, but spread as rumors because the official version of events is unclear or clear evidence is either withheld or missing. It's a human trait to try and fill in the blanks, and the more extreme memes spread faster than the mundane ones, meaning that the wilder conspiracies get around much faster than more plausible explanations.