UFOs are often convenient cover for secret Re:Carl Saganflight tests.
That gives the government an incentive to encourage UFO nuts.
A lot of the cold-war-era "conspiracy theories" sound like "second cover" stories. That's a psychological technique for diverting investigation into some large-enough-to-be-worth-the-effort secret project. Works like this:
Plant TWO cover stories. The first is plausible but misdirection. The second is fruitcake-nuts (but ideally has aspects that look attractively like actual artifacts of the project being hidden). Somebody investigating what is going on first hits the first cover. If he accepts it, fine. If he notices it doesn't quite fit and digs deeper, he finds the obviously screwy second cover. Oops? Now what?
The tendency of the more rational is to reject it - but bounce back to the first cover and give up there. The less well-hinged may report the second cover (much to the glee of the security people). Few are going to keep digging past both to discover some approximation of what's really going on - and if they DO get there and talk about it in public, if they happen to have said anything related to the second cover story (or even if the HAVEN'T), they can be debunked by painting them as having accepted the self-evidently tinfoil-hat-grade second cover story and propagating a variant of it.
The "conspiracy theories are always wrong and insane" meme is very convenient for this as well (as it is for any actual conspirators B-) )