Why are UFO stories mainly from USA?

Potentially culture. There's a lot of supernatural events that are reported everywhere (not saying they're real). It's entirely possible that American culture means those kinds of events are more likely to be interpreted as UFO sightings as opposed to another kind of event.

A lot of sightings are also aircraft tests (IE: B2 bomber), so it could be that other nations test less alien looking aircraft. The New Generation Fighter in the EU looks like an F18, so a mistaken identity is probably going to skew to "fighter jet" and not "ufo".

I don't know if your assumptions are true[0], but..

Maybe because UFOs were invented in the USA - it was a craze/hysteria started in 1947. After Kenneth Arnold said he saw flying saucers that year near Seattle, people started seeing them everywhere.

"Arnold's account was first featured in a few late newspaper editions on June 25 ... Starting June 26 and June 27, newspapers first began using the terms "flying saucer" and "flying disk" (or "disc") to describe the sighted objects. ...

In the weeks that followed Arnold's June 1947 story, at least several hundred reports of similar sightings flooded in from the U.S. and around the world—most of which described saucer-shaped objects. A sighting by a United Airlines crew of another nine disk-like objects over Idaho on July 4 probably garnered more newspaper coverage than Arnold's original sighting, and opened the floodgates of media coverage in the days to follow.

Bloecher collected reports of 853 flying disc sightings that year from 140 newspapers from Canada, Washington D.C, and every U.S. state save Montana. ... The most famous UFO event during this period was the Roswell UFO incident, the alleged military recovery of a crashed flying disk, the story of which broke on July 8, 1947."

https://en.wikipedia.org/wiki/Kenneth_Arnold_UFO_sighting

[0] i.e. Before asking Why are X mainly from Y? make sure that X mainly are from Y. Even if they are, maybe doing so will answer your why question.

My completely uninformed guesses:

- USA is great at exporting its culture. You don't hear about Estonia's UFOs because you just don't hear about Estonia. I know a lot of UFO stories about my home country - which is not Estonia - and about USA.

- USA people seem to be very adept at believing every kind of conspiracy - of course, not everyone. I mean, just read USA news from 2020. I see a lot of similarities in people who would believe UFOs and people who would believe the new things. UFOs are not as viable in terms of conspiracy due to the omnipresence of recording devices now. This may be related to the poor educational system in addition to the prevalence of religion and other supernatural beliefs that are widespread.

- USA possibly experimented a lot with weird spy flying machines during the Cold War. I bet a lot of these UFOs were weird airplanes under test.

Maybe because some kinds of news tend not to be considered newsworthy if it doesn't come from "interesting" countries. Or tend to not to even gather local interest, fizzling out before being picked up elsewhere. E.g. in South America there are (a few?) very active ufologists, purported UFO sightings, people claiming abductions, communications or contacts with aliens. Sometimes they even manage to capture some lights in the sky. But they are (or were as of a decade ago) not taken particularly seriously by local press, so I'd expect the vast majority of these claims, specially those with a strong local flavor, not to bubble up to international news.

Does your impression come from general news or reading UFO sites? I'd expect a lot more (badly) reported incidents from other countries in specialized media.

just like almost every superhero is from USA and every bad cop is from USA, every bad guy is from Middle Eastern

The rest of the world didn't have area 51 military testing 50+ years ago producing sights no one had ever seen before. In a desert with clear skies.

That triggered a cultural phenomenon. Tons of people like to make fake photos and videos and tons of people want to believe aliens are visiting us.

Space race? Hollywood? Area 51?

There are plenty of UFO sightings outside the USA. You just don’t hear about them if you don’t speak the local language.

makes more sense to ask aliens riding the UFO's, doesn't it.

Because USA think they are the center of the world