How Do Loyalty Programs Make Money For Airlines?

Airline loyalty programs today are ubiquitous. They are advertised almost everywhere in airports, online, on social media, and even on flights themselves, with many airlines offering passengers special signup bonuses for their mid-flight decision to apply for a credit card. Carriers will even go so far as to have flight attendants recite sales pitches at 35,000 feet, hoping that they might be able to earn a commission should a passenger sign up as a result.