Not to be that guy, but if the majority of sightings coming from nations capable of producing advanced military aircraft... well perhaps the aliens thought the F-15 was badass and wanted a closer look?
US sightings get publicised best due to the nature of international media (wide distribution of US films, TV and books)... But there certainly are other places claiming to see them. There are a lot of supposed sightings in Chile for example, which doesn't have a huge air force.
