Japanese zoning restrictions also contribute heavily here. It's not just that higher-density residences are available, it's that you can put them nearly anywhere. According to http://urbankchoze.blogspot.ca/2014/04/japanese-zoning.html, not only are there vastly fewer zones (12, nationally), Japanese zoning laws specify only the "maximum use", instead of exclusive uses. So anything zoned for commercial use, for example, could also have neighborhood stores and residential.