I don't know if your assumptions are true[0], but.. Maybe because UFOs were invented in the USA - it was a craze/hysteria started in 1947. After Kenneth Arnold

I don't know if your assumptions are true[0], but.. Maybe because UFOs were invented in the USA - it was a craze/hysteria started in 1947. After Kenneth Arnold said he saw flying saucers that year near Seattle, people started seeing them everywhere. "Arnold's account was first featured in a few late newspaper editions on June 25 ... Starting June 26 and June 27, newspapers first began using the terms "flying saucer" and "flying disk" (or "disc") to describe the sighted objects. ... In the weeks that followed Arnold's June 1947 story, at least several hundred reports of similar sightings flooded in from the U.S. and around the world—most of which described saucer-shaped objects. A sighting by a United Airlines crew of another nine disk-like objects over Idaho on July 4 probably garnered more newspaper coverage than Arnold's original sighting, and opened the floodgates of media coverage in the days to follow. Bloecher collected reports of 853 flying disc sightings that year from 140 newspapers from Canada, Washington D.C, and every U.S. state save Montana. ... The most famous UFO event during this period was the Roswell UFO incident, the alleged military recovery of a crashed flying disk, the story of which broke on July 8, 1947." https://en.wikipedia.org/wiki/Kenneth_Arnold_UFO_sighting https://en.wikipedia.org/wiki/Kenneth_Arnold_UFO_sighting [0] i.e. Before asking Why are X mainly from Y? make sure that X mainly are from Y. Even if they are, maybe doing so will answer your why question.