Why are there airport delays?

An interesting article from Slate about why your flight is always delayed. Hint: it ain't the weather.

The most important source of late flights at big airports is completely within the major airlines' control. In fact, the evidence shows the major carriers are creating the delays on purpose. ...
Mayer and Sinai's study also identified the real culprit: the deliberate overscheduling of flights at peak periods by major airlines trying to increase the amount of connecting traffic at their hub airports. ...
American Airlines, for example, uses O'Hare as a hub and schedules a cluster of flights to arrive there from the east in the earlier afternoon. Another cluster leaves for points west and south soon after. In the 30-minute period between 2:45 p.m. and 3:15 p.m., American has scheduled about 18 takeoffs, not counting its regional flights. That comes close to maxing out the airport's capacity, without any other airline. Other airports are even more extreme. Continental has seven flights scheduled to depart during the exact same minute (11:45 a.m.) out of Newark, as well as almost 20 other flights in the surrounding half hour. Some of these flights leave late more than 80 percent of the time. The major airlines know perfectly well that these hideous statistics are inevitable.

They go on to report the obvious solution, which is to schedule flights more evenly throughout the day. The airlines won't do this, howevs, because customers won't pay for tickets with big delays (layovers) already built in. They figure that customers will continue to buy tickets that look efficient, even though there is little to no chance that flight will leave on time.
I fly into O'Hare every time I go home to Wisconsin, and I now avoid American Airlines for precisely this reason. Every time I fly with them, I get delayed. The airline business is one of the most competitive, so I took my business elsewhere. I fly Independence Air now, and while there are still delays, they are much more manageable. American Airlines lost my business.

Annie, the tragedy of the commons is also mentioned.