The other factor is that old houses are downright unsafe. This is a place that regularly experiences earthquakes, sometimes big ones, and construction standard

The other factor is that old houses are downright unsafe. This is a place that regularly experiences earthquakes, sometimes big ones, and construction standards have improved over time after learning lessons from bad earthquakes and with improved technology. So no one wants to live in a 50-year-old building that's likely to collapse when the next big earthquake happens if they can help it, and so buildings tend to be demolished more frequently and replaced with newer ones.