> I've always wondered why UFOs are only a USA phenomenon.

They're not. There are however two factors greatly biasing your perception of UFOs. First, it takes a fair amount of effort to track down information in foreign language sources, so in general there's going to be a lot less English-language information on Chinese, Russian, or Japanese reports of UFOs. Second, militaries are going to both generate more UFO reports (thanks to military research programs that go awry, e.g., the circumstances behind the Roswell incident) and be a lot more open to collecting and investigating them (because of their ability to indicate the previous point). The US military being the largest and most well-funded in the world is thus going to generate the largest UFO report halo around it.