Not sure about Korea, but Japan's case is more interesting than simply adjusting for population: the government strongly incentivizes new buildings every few decades. There is also a culture of viewing housing as a consumable, which is the part that sticks out to me the most.

Demand for the big cities is still increasing pretty much everywhere, even if some countries are seeing net decrease in demand on the whole. Also remember that supply deteriorates. The number of "ready to move into" homes can decrease over time without maintenance and rebuilds.