Isn't China's urbanization rate quiet low at the moment.
Better to build some reserve if you need to move people from rural to urban areas.
So if you have to move hundreds of millions of people into urban areas
its probably a good thing to have some housing reserves build up to facilitate those migrations.
China has been doing that for 3 decades. The target is a 60% urbanization rate. That's why there are so many "empty apartments". Because they are build and it takes a year or two to fill them up, so it looks like they are "empty". I lived in two such developments during the years that I lived in China. It's interesting to see the policies used to fill them. And to come back to the place a year later, when it is crowded.
Most developed nations have like a urbanization rate of at least 80+%. To reach developed nations urbanization rate china would need to move like 300 million people into urban areas, doing some quick maths.