Conspiracy junkies get off on the perception that everyone else is wrong. What they believe in must be, therefore, incredible.
To fill this emotional need (it certainly isn't an intellectual one), they assume certain things about human nature that, surprise, most others don't. That humans beings are actually good at keeping secrets. That large numbers of human beings are capable of entering into a functioning conspiracy. That human beings are nearly perfectly competent. That members of such a conspiracy are capable of acting, indefinitely, without conscience. And on and on...until the next assclown puts up a website revealing that there really is, in fact, a B17 on the moon.
To this end, they employ a couple of logical, or illogical, devices. One is massive filtering of publicly vetted evidence, you know, all the stuff the government 'wants you to believe', in favor of those few dots that can be connected by a creative mind to reveal the dark secret within. These lies are substited with 'conclusive proof' based on something that DIDN"T happen or that is MISSING...contraary to any Logic 101 text, of course. This 'proof' is usuallly bsed on one or two grainy photos (hand picked out of hundreds of others that constitute the body of lies, of course)