USA is considerably new country and built with no distinctive, coherent ancient history and mythology. As such, its people often grab every opportunity to produce a mythology they can consider their own. It makes them perceive as unnatural/supernatural things we, citizens of old world rationalize, or brush off as of little value.
USA is an experiment by whatever the “aliens” are, so it’s natural for them to supervise it with more interest than the rest of the world.
There are no aliens, it’s just USA itself testing new technologies of theirs.
Other superpowers are very secretive by default and they simply hush-hush every sighting they can.