Culturally, Godzilla in Japan very much came out of the fear of nuclear weapons and radiation post-Hiroshima and Nagasaki -- a monster awakened and empowered by nuclear weapons [1].
I've never been super clear on what the cultural reason for aliens <-> USA is though. Although I suppose it's different in that Godzilla was an intentional fictional creation, while the fascination with aliens hopes they're actually real.
There have been reports of aliens, alien crashes and recovery, etc. all over the world[0]. The reason it's primarily seen as an American phenomenon is that the whole culture and mythos originated in the US, first in the late 1940s with the Kenneth Arnold sighting[1] (where the term "flying saucer" originated) and later Close Encounters of the Third Kind and Whitley Streiber's Communion codified the "grey alien/abduction" motif (although there were prior stories like the Pascagoula abduction[2], notable for the aliens described as looking nothing like greys) and of course the X-Files. The rest of the world likely absorbed the UFO archetype through cultural osmosis. Also Americans simply aren't going to be aware of UFO reports from other countries unless they go out of their way to look for them.
In 1979, Ricardo Vilchez, one of the project team members, sent the photograph to an organization in the United States called Ground Saucer Watch (GSW) which analyzes these types of cases. GSW determined that the image was true and unadulterated, reported CRHoy.
Sierra [a UFO researcher] said the photograph was analyzed in France and the United States by different organizations that studied the case, and all confirmed the object’s existence.
We only have word of mouth from UFO researchers citing unnamed "organizations" to verify this. I'd want to see an independent, accredited and non UFO related team actually study the negatives and such. Otherwise it's just another picture on the internet. It is a really good picture, though. Classic shiny flying saucer.
Although I am concerned about the shape of the reflections in this "high res" version[0], they don't look right to me.
There are very prosaic reasons for the popularity of UFO sightings in the USA.
1. The gdp per capita per cloudless land area is far higher than anywhere else. Africa and Australia have lots of empty sky but relatively no-one is out and about during night time. Plus USA does some have some interesting weather. So the opportunities to see anomalous (yet very terrestrial) phenomena is high.
2. The concern that comes from being the richest and most powerful country in the history of the world, that others are trying to infiltrate. (I was almost going to use fear or paranoia instead of concern, but some of the concern is warranted)
3. USA is the center of world culture. Everything American is amplified via Hollywood and music.
I've never been super clear on what the cultural reason for aliens <-> USA is though. Although I suppose it's different in that Godzilla was an intentional fictional creation, while the fascination with aliens hopes they're actually real.
[1] https://en.wikipedia.org/wiki/Godzilla