Japan has a much better mass transit system and a culture that does not embrace the automobile like the US has. Plus, Japan has far more access to fresh water that California does. And Tokyo doesn't have that many skyscrapers. In fact, for a city of its size it has surprisingly few.

Even if we could magically upgrade every SFH to a multi-family building we'll have very unlivable cities with infrastructure falling well short of comfortably supporting the resulting population. So a realistic and worthwhile path to 100M would be a long and very expensive process.