I imagine UA is making a mint on cargo. I remember years ago hearing from someone close to the matter that (even with full cost allocation) when BA was running SAN-LHR (or SAN-PHX-LHR) the cargo business/demand was so strong that BA turned a substantial-enough-to-be-worth-a-LHR-slot profit even with 0 passengers on board. Every passenger above 0 was essentially pure profit... The demands for and fees charged for US/Europe air freight have only gone up since then.
Many years ago CNBC, IIRC, did "a day in the life" of AA and one of the most interesting things from that show was they followed a JFK-LAX fight with a complete income/expense breakdown... IIRC their net profit was around $5k...Would love to see that updated or revisited.