It's fairly simple, actually.
The Japanese are racist as fuck, first of all. Ignore how people talk about how nice they are: The Japanese will not like you unless you conform to them- they do not conform to you. The Japanese are proud of their heritage and basically refuse to accept that they are anything but a small pawn in the global community.
Additionally, most Japanese don't know what real Americans are like, they just guess from movies and TV shows. Face it: If we judged our own culture from our popular culture, we'd be slanted toward blonde haired blue eyed models too.
tl;dr: Japanese people are stupid.