It’s speculative investments, housing as assets instead of, well, housing. In almost every major city in the west there is an astonishing number of empty apartments. In my hometown of Berlin there is essentially one large corporation that owns most of the city as investment. Also, new housing is constantly being built - but not for (average) people to live in it.
You may also recall that the whole thing came crashing down in 2008? Or have we just forgotten what happened there and the effects it has to this day.