It originated from the Cold War, and is a consequence of American Propaganda that both covers up intelligence and military action inside our own borders (the original area 51 conspiracy was started and propagated by the US military) as well as our inate belief that we are the worlds biggest and most powerful superpower, therefore the only things that could invade our airpsace must be extra-terrestrial.

Edit: They didn't conspire to invent the conspiracy, but didn't outright deny it either, since it provided them with a convenient distraction