I think you may be on to something about it being an intrinsically 20th century phenomenon. It may work out like Spritualism, where the movement ran strong for a relatively brief period and then petered out under the withering light of popular scrutiny. Its effects on our culture are still omnipresent, but very few people subscribe to it nowadays because the idea that the dead are with us and want to communicate to us - but only for the sake of performing party tricks and sharing the most tedious of banalities - is hard to really take seriously.

The UFO meme's not so different. Super-advanced races using their incomprehensibly superior technology to fling themselves across the near-infinite vastness of space are visiting us in order to. . . prank aircraft pilots, share tedious banalities and diddle butts. The word 'risible' again comes to mind.