I’m dropping in on the thread at @CountryBreakfast@lemmygrad.ml’s recommendation. The source of the statistics is an organization located in Washington state in the United States. The online form to report a UFO sighting is only in English. If you look at the map, most of the countries that have high per capita reports of UFO sightings also have a high rate of speaking English. Not every country follows that - notably Mexico and India - but most do. So while there may be cultural factors, I think sampling bias (linguistic and country of origin) is likely the largest source.

Edit: Also of note is the US’s history of advanced aircraft. During Jimmy Carter’s run for president, he promised to release all UFO-related records if elected. Immediately upon entering office, he broke that promise. There is speculation that he broke that promise because many of the sightings were stealth plane test flights. It lines up with what we now know about the timing of their development, location of certain bases, and the location of some sightings.