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.