Everyone has to live somewhere; fitting 200 people on a small but tall, fancy condo plot frees up 200 people-units elsewhere. That's how supply and demand work.
Except that somewhere isn't implicitly in the vicinity of the new development. That somewhere could be in the suburbs, another city, a dorm room, a room in a parent's house, or any other options which doesn't have any real impact in the availability of housing in an area.
edit: more on that here: http://www.slate.com/blogs/moneybox/2012/12/11/filtering_vs_... .