Everyone ignores this possibility: the aliens are future Earthlings who have mastered time travel.
(Some people think time travel is impossible but aliens zipping around the universe are plausible.)
If time travel were possible we would be inundated with tourists from the future.
I’ve kind of always believed this is the most likely possibility. Aliens are just future man just traveling back to their past via time travel
Or beings not from other solar systems in the galaxy, but alternate dimensions. Might explain why the UFOs seem capable of violating the laws of physics as we understand them, and why they often seem insubstantial.