Because land is an awful example of a free-market commodity. It's not like you can set up a land factory to drive prices down (outside the Netherlands and Singapore I suppose...), and it's something everyone needs to live an even vaguely decent life.

And even worse, someone who owns land doesn't need to do a damn thing to make it increase in price; it's the businesses and people living around it that do that. So money sitting in real-estate is far less useful to society than money invested into productive businesses.