And, prior to the meme of the "flying saucer," UFO sightings often involved airships[0].

And stories of abduction by aliens often involved those aliens claiming to be from Mars or Venus before it was commonly known that those planets didn't harbor life, much less advanced civilizations.

And the archetypal "grey alien" didn't really become the template for modern alien abduction stories until Whitley Strieber popularized them in a book - many descriptions of alien encounters until that point were of "Nordic" human type aliens.

So yeah, either aliens are being careful not to appear more advanced than popular culture would allow (maybe to the point of appearing as angels and demons to humans when their view of reality was entirely dominated by religion and not science) or much of the UFO phenomenon is basically a meme.

[0]https://en.wikipedia.org/wiki/Mystery_airship