I would say it's a "Big City" thing. New York gets the same rap. It's what I call "Gerbil Syndrome". Gerbils become very aggressive and combative if too many live in a small area. It's essentially a fight for resources.
Country folk are friendlier...this is true in the US as well and probably most countries.
Though I also think Japan uses the "polite" thing as a fine art to debase and or insult others and get away with it. Like when you say something mean online and add a
at the end or the tag line "No offense".