Because the major export product of the United States is American culture. UFO/alien abduction/government cover-ups are kinda baked into American popular culture at this point. Just the idea that the government is simultaneously incompetent yet able to successfully hide far reaching evidence of alien visitation is as American as baseball and apple pie.
I’ve no doubt people are looking up at the night sky all over the world and seeing things that they think are possibly alien craft. I’ve also no doubt that there are stories and folklore about abductions in every country on earth. But American culture is insidious and infects just about everything. This forces every other event to be reframed in reference to the American UFO phenomenon.