Because they build the cities/apartments before there is really any demand for them. There isn't much in the way of trade or industry in these ghost towns so there isn't much of a reason for most people to move to them in the first place.
I believe the thinking was that all of the cash-hoarding - "for-the-family", that's been occuring over the previous generations would finally be cashed in by this last one and there would be this huge, western-influenced group of twenty-somethings doing something that no other generation has done previously - actually move away from their parents/grandparents/et al home! To some extent, it is occuring. But I suppose Central Planning must've thought all of them would do it all at one time, lol.
