I'm of the opinion they are part of the same phenomema as Aliens, Flying saucers etc, and they appear to people using whatever frame of cultural reference is uppermost in the psyche of the witness. There are lots of similarities between Aliens, and Fairies. Greenish skin, pointed ears, gifts that disappear in the morning, missing time, bright unexplained lights, abduction,
strange sexual practices, cross-breeding programs, 'recovered' memory, strangely sunburnt skin of victims, the list goes on. In the USA, 200 years ago, people reported seeing strange flying 'ships' in the air. These ships were like seagoing ships, with sails, and there was even one report of one that got it's anchor entangled in a Church Tower.
But there is never any physical evidence left behind.
The Peri of ancient Persia would abduct humans on flying carpets, Khidr, the Green Man, would appear to people in the Middle East, and they would suddenly find themselves hundreds of miles away, with no memory of how they got there. The worlds myths are full of such encounters with these beings, and they all share threads of similarity.
But yeah, I seen them, and it still beats the shit out of me!