Why cant japans cities expand farther

Japan is experiencing a "super-aging" society both in rural and urban areas ... The dramatic aging of Japanese society as a result of sub-replacement fertility rates and high life expectancy is expected to continue. Japan's population began to decline in 2011. The government will have to spend more on healthcare, and that, coupled with a shrinking workforce and tax base, is a recipe for economic stagnation. It also means, among other things, that there will not be enough young people to care for the elderly