It's simple, but ultimately self defeating economics. Airport owners want to spend their money on retail outlets, not runways. In fact, more runways means faster flow-through of people. This is bad. More congestion means more bored travelers wandering around the terminal (i.e shops), and it means peak periods last longer during the day. This is good. The fact that aeroplanes are burning $ in holding patterns is an airline scheduling issue, passengers will still come, and it's better to have the terminal constantly busy rather than suffer peaks and troughs.
Not sure if the long term consequences of this mentality e.g airline receivership,
have been fully considered though...