First of all, I've seen Japanese (who are strangers) strike up conversations on occasion in Tokyo: at the coin laundry, at the bakery, on elevators, and so on. It's not like a social taboo or something. But yes, in general, people keep to themselves.
That being said, in my experience, people in New York and Los Angeles also keep to themselves. My instinct is that the apparently introverted nature of people in Tokyo has nothing to do with Japanese culture and everything to do with the fact that it is a big city in a developed country. Perhaps someone from London can bring in another data point?
If my experiences in rural Japan are at all normal, Japanese in rural Japan are at least as talkative as those in the Southern US (where I'm from).
