Depends where. In an area like downtown Chicago, you want to be in certain spots for the prestige of the area. A "square lot nearby" would, simply because every square lot nearby is filled already, take you outside of the area. However this triangle lot nobody wants because it's a goddamn triangle is relatively cheap and where you want it. Thus, that's what you build. Why you build a triangle building instead of a rectangular one, presumably, is looks.