Someone else hit the nail on the head - the tickets are too cheap and the cost base is too high.
The customer end of the business involves filling planes with passengers paying $50 - 500 each for their ticket.
When you have a cost base like Ryanair or EasyJet, then you just about make money. With a cost base like AA, you lose money hand over fist.
You're spending more than you're taking. It's not complex.