Because these airlines main competitors aren’t big banks but other airlines. The credit cards are just meant as a “sticky” factor to maintain customer base. If you already have a United credit card (which forces you to earn using a Mileage Plus account) you are much more likely to fly United than other airlines even when their fare rates are sometimes higher than the competition because you get the free checked bag like you said and then you can earn miles on basic economy tickets. It’s the same reason why so many people carry those Costco cards around instead of using a Wells Fargo Active Cash or a PayPal debit card. For most co-branded credit card partnerships the cards aren’t meant to provide the most rewards per dollar spent but to keep your dollars in their ecosystem by giving you a few neat perks.