There are probably deep psychological reasons behind virtually every common social phenomenon, Bee-dub...not just the zombie fad. There may even be some behind your last post, for example. ;-D

There has indeed been an enormous popularity worldwide in the last few years for zombie films, probably moreso than ever in the past, although it's happened before. The majority of those films do emanate from the USA, and the USA culture is the biggest cultural influence in the world in our era, spreading itself through popular music, movies, marketing, war, and advertising...so to ascribe the zombie film phenomenon primarily to USA culture is probably accurate, which is not to say that it wouldn't be getting popular elsewhere too...like other things emanating from USA culture.