From NBC News:

The question is one you may have asked yourself while standing in line at the  airport or when trying not to scream in frustration while sitting on the tarmac  waiting to take off: If airlines know there will be a surge of passengers during  the holidays, why don’t they add capacity in the form of extra flights?

While it makes sense in theory, the reality is far more complicated.

Despite mergers in recent years and the retiring of older, less-efficient  planes, carriers have nowhere to turn for a dozen planes the week before  Thanksgiving.

That’s not how the commercial airline business was set up to operate,  industry experts said.

“It’s not realistic to think airlines could add planes for a few days,” said  Josh Marks, CEO of airline consulting firm masFlight. “The airlines would love  to have the flexibility to add capacity, but the system is not set up for it.”

There’s no Hertz for airlines looking to add a Boeing 737 for a week or  two—leasing companies don’t operate that way. And because airlines have  transformed themselves to be as lean as possible, they don’t keep “extra” planes  sitting around.

Even if the airlines could get their hands on, say, a dozen planes, who would  fly them or serve passengers? Carriers are already fully staffed and have crews  tightly scheduled. Most airlines have recalled all the pilots they furloughed  before the recession.

Even if some idle pilots were available for temporary hire, existing pilots  would be up in arms that “rental crews” were brought in to handle a particular  route.

System stretched to the max

On top of that, airport takeoff and landing slots are already jammed at many  of the busiest airports.

Take LaGuardia Airport in New York. Even if an airline wanted to add three or  four more flights out of the airport on the Tuesday before Thanksgiving, when  would they take off? LaGuardia is already slot-controlled, meaning the Federal  Aviation Administration limits the number of takeoffs and landings per hour, as  is Washington’s Reagan National Airport.

If one airline wanted to add three flights on that Tuesday, its competitors  would want to match it and add flights. Soon, the most congested airspace in the  U.S. would be even more so. You get the picture: Gridlock, raised to a whole new  level.

Increased air travel would mean more users of facilities whose infrastructure  is already threadbare.

“Our airports are so clogged up that we’re going to face Thanksgiving almost  every day in a few years in every airport,” Roger Dow, president and CEO of the  U.S. Travel Association, said recently.

Tighter capacity, higher fares

The argument for adding more planes also ignores one crucial factor to  airlines: profitability.

Keeping  capacity in check in recent years has allowed carriers to turn a profit. They  now fly fewer planes with a higher percentage of passengers. While we may not  like that there are few choices or discounted fares, that’s exactly what the  airlines want.

On the busiest days of the year, like the Sunday after Thanksgiving, airlines  will run their fleets faster and add as many flights as possible. According to  masFlights, Delta Air Lines will run 10.5 percent more flights on Sunday, Dec.  1, than it did on Sunday, Nov. 14.

“When it’s busy, the airlines are at capacity,” Marks said. “There’s no  wiggle room to add more.”

Source: http://www.nbcnews.com/travel/why-dont-airlines-just-add-more-flights-holidays-2D11663461