It's a loss leader
It's like a lot of Las Vegas hotels. You don't make a lot of money from the people in the rooms, but you hope they spend more in the casino or restaurants.
If you don't sell someone an airplane flight, UA can't charge for luggage and change fees. It won't benefit from non-refundable, un-used tickets. It can't charge for phone reservations fees. There's less incentive to have the UA Chase card. They can't market car rentals and hotel rooms. There's less incentive for other carriers to pay UA to market their flights. Less people on expense accounts in Clubs buying Premium Wines/Liquors/Beers. etc
So the less you spend on passengers during the flight experience, the higher your profit margins are.