There has been a lot of speculation about why the empty cities, with a lot of reasons proposed. I saw a documentary on youtube that explained it well. You may recall that china had a one child per couple policy for a long time. For cultural reasons chinese people want a male child, so they aborted most of the females. Now the have a huge population of single males of marriageable age who are in competition for the few females available. To be considered for marriage, a chinese man has to own a house or apartment. And that's who have been buying all of those empty apartments. The documentary explained it well, and it makes more sense than any other possible reason to me.