An awful lot of it I think…
It’s been quite well documented, and there’s been a large amount of speculation over the years.
Aside from the cover-up of technology that they engaged in, I have a suspicion (we’ll call it a conspiracy theory) that certain authorities have conspired to nurture UFO conspiracy theories so as to divert some of the public’s attention from more down-to earth conspiracies — less than fantastic covert operations that government agencies were involved in.
That there may be alien invaders from Altaire 4 is more likely to engage the public’s imagination and fascinate than something so mundane as testing and spreading drugs to the citizenry, or covertly supporting a coup in a ‘third world’ country.