It isn't the case. It's a simplistic gloss on a complex finance outcome.

Some flights make money.

Some flights lose money.

Some finance structures make money while looking like losses to acrue tax benefits for other activities.

Sometimes the money is being made by holding companies not operating companies. Sometimes the assets are worth more as spares than operating.

All companies are complex. I do not think "flights don't make money" is true for all airlines, all flights.

Because people take "airline X makes $50k profit, and makes $55k off of the credit card, so therefore it makes all money from credit cards" which is true from a certain accounting point of view, and also entirely false, in that it's all accounting tricks and the credit card would be worthless without an airline.

Not to mention that loyalty programs and credit card bonuses don't exist in Europe.