Why do the Japanese have a group-based, unindividualistic culture?
The traditional Japanese village was both involved in rice farming, where cooperation over water supply etc. is vital, and isolated by mountain and government policy from surrounding towns. Hence, the old lady in your Tokyo apaato block gossips and interferes like her ancestors did in the paddy fields 2 or 3 generations ago. It should also be pointed out that many parts of Europe, e.g. the Med and the Balkans (Spain, Turkey, Southern Italy etc), resemble Japan at least as much as they resemble Northern Europe on this point.