It's because they make just as much money selling airline miles to credit card companies as they do selling seats to passengers.

https://www.bloomberg.com/news/articles/2017-03-31/airlines-make-more-money-selling-miles-than-seats

In many ways, the Big Three U.S. airlines have organized themselves into two distinct businesses. There’s the traditional activity—the one with jets—which involves pricing seats for as much as possible, collecting a bag fee, and selling some food and drinks while keeping a close eye on costs. The other business is the sale of miles—mostly to the big banks, but also to companies that range from car rental firms to hotels to magazine peddlers.

The latter has expanded so much that it accounts for more than half of all profits for some airlines, including American Airlines Group Inc., the world’s largest. “Airlines are earning upwards of 50 percent of [income] from selling miles to a credit card company, which we believe is a great business to be in,” DeNardi wrote on March 20, boosting his target prices on American and United Continental Holdings Inc. by $30, raising his outlook for Southwest Airlines Inc. by $15, and adding $10 for Delta Air Lines Inc. shares. He cited the likelihood that airlines will begin disclosing more information over the next year or two. Stifel also upgraded its target share price for Alaska Airlines’ parent to $145. That stock traded at $93.66 on March 30. DeNardi argues that more transparency about loyalty plans would also pressure airline executives to further improve profits in their core business—namely flying.