Oil prices have fallen by about half since June, making it much cheaper just in time for drivers to fill up their cars for Christmas travel. But the decline in oil prices has had made no perceptible difference on the cost of flying. Fares are up from earlier this year, and many airlines are still levying significant fuel surcharges on the tickets they sell.

There are many reasons airlines have not lowered fares to reflect the decline in oil prices. Some of these companies are still paying high prices for fuel themselves because they have to abide by long-term contracts. But the biggest reason airlines are not passing on lower prices to consumers is that they don’t have to.

Demand for air travel is strong, and a series of megamergers has significantly reduced competition in the industry. The four biggest airlines in the United States — Delta, Southwest, United and American — control about 80 percent of airline capacity, down from 11 companies as recently as 2005. For most travelers, that has meant higher prices and jam-packed planes.

After suffering through years of losses and bankruptcies, airlines are prospering. The International Air Transport Association says that the industry’s profits will grow by about 26 percent, to a record $25 billion, next year. The association says North American airlines now have profit margins higher than in the late 1990s, before the 9/11 terrorist attacks devastated the industry. Airlines for America, a trade group, says its members are investing their growing profits in new planes, better amenities for passengers, employee profit-sharing and dividends.