Forbidden Planet: Forbidden UFOs
The Krell, who died out 200,000 years earlier in the film's scenario, had a culture derived from millennia of technical, artistic advances.
The idea of a crashed flying disk is ludicrous on the face of it.
But, for the sake of argument, let's assume that lightning or some other unexpected Earthian surprise brought down a space-traveling craft and its crew.
Would the crew not have accoutrements of an advanced kind, artifacts too futuristic to be understood by those recovering their downed ship?
The problem with UFOs and Roswell is that the indications and reports by alleged witnesses are too mundane to support an advanced race of extraterrestrial travelers.
(Moreover, the Earth, as I keep noting, is too insignificant to attract a space-traveling species: the galaxy and universe itself have more, many more, interesting places to visit, despite our desire to see ourselves as a special creation on a special planet.)