And its the first time i hear someone say that 30 years limit for buildings is a good thing. In practice they use shitty and cheap materials, have no isolation so its too hot in summer and cold in winter and it looks like shit. They dont build anything to last and its not a good aspect since its wasteful.
As for bedroom communities without commerce, I would attribute that to: (1) an excellent public transportation infrastructure, and (2) an overall migration of livelihoods to Tokyo/Osaka-scale urban centers. My experience (in Nagoya) is that there are offices and light industry mixed with residential in every direction until you reach 里山. In fact, many people that I know operate businesses out of their homes, including light manufacturing, restaurants, etc.——something you cannot do in most American cities.
And finally, I can certainly understand your view on the "disposable" nature of housing here, but I still think it makes good sense. This is earthquake and typhoon country, so the codes change with advances in technology. The industrial nature of the housing business here drives the cost way down. In America, residential construction projects are notoriously difficult, over-budget, and late. Compared the efficiency of scale of a company like Sekisui, American contractors are an unfunny joke. I have seen homes built in Japan with a crew of no more than three or four men, plus a few traffic guards, in just a few weeks. And they work quietly and keep the area clean. I can build a new house in my city in Japan for around US$150,000——something that is completely impossible in most American cities. (The price of land here in another matter, of course. It's very expensive.)
I will never understand why Americans pour money into old houses. Even 伊勢神宮 is rebuilt every twenty years, and it's more than two thousand years old!
A lot of Americans are under the illusion that when they buy a house, most of the value is the house. It's actually the land (bought as a package deal), but the perception that the house is valuable drives the behavior that's confusing you.
That makes it possible to have much denser towns.
They're one step closer to the ideal.
This way of living might offend some people's aesthetics or their idea of what they deserve, but it works very well.