They don't really "fail." Airlines do pad their schedules for known congestion issues. DCA-LGA is about half an hour of flight time; once with a phenomenal tailwind we made it in 18. Nevertheless, while an early morning flight will be scheduled for an hour, an early evening flight will be an hour and a half, with the knowledge that you'll spend 20 minutes on the tarmac at DCA waiting for permission to head to New York, then another 20 minutes at LGA waiting for a gate to open up. Airlines will also alter schedules for known big events, say, using bigger planes on certain routes for the day before Thanksgiving or the day after the Superbowl.

But an airline has to watch its costs. Even if a flight is delayed 50% of the time it still might be better for business not to add padding, because you'd lose potential frequencies and customers to competitors and have to pay your staff for more hours for the 50% of time it isn't delayed, and hear complaints from people arriving too early for their carpool.