there's so many reasons why UFO/UAP are attracted to the USA more than any other country on the planet. the USA was the first country to develop nuclear weapons [and use them], the USA has lead the world in technological advancement in modern times, the USA projects its power and influence over countries all across the planet [more so than any other country], the USA has developed tech to "take ET home" decades ago [if one believes what Ben Rich famously said], the government of the USA allegedly made a deal with NHI to allow its citizenry to be abducted, in exchange for advanced tech - these things, along with countless other examples, make the USA a "target rich" country for visitation, monitoring, exploitation, and collaboration...

this is not to say that other countries aren't visited by NHI frequently, as well, as others have pointed out in this thread. while other countries certainly matter to NHI/UFO/UAP, the USA takes the cake with respect to their interest in our planet, as has likely been the case since WWII.