The incentive is that people who own land (or even people who own existing buildings) can make money if they rent it out. It boils down to simple greed; as Ada

The incentive is that people who own land (or even people who own existing buildings) can make money if they rent it out. It boils down to simple greed; as Adam Smith said, "It is not from the benevolence of the butcher, the brewer, or the baker that we expect our dinner, but from their regard to their own interest." Adam Smith's big contribution was showing that, generally, if we assume people are greedy and will look out for themselves, we can still end up with a fair system. Lasseiz faire economics has a much better track record in that regard than the alternatives that try to ignore greed. You can set things up based on how you want them to work, or you can set them up based on how they really do work.