Because the product is often not what we think it is anymore.

Surely the airline company makes money with the actual flight... but also they might start to make money with selling private data. Maybe they even start to make MORE money due to a commission on car rental, hotel, tourist attraction, etc than the actual flight.

The "product", due to pressure from competition, enshittification, novel alternatives, is changing and we as customers don't necessarily notice it. In fact the company is probably doing its very best so that we don't and can thus benefit with side effects that we are not aware of, otherwise we would ask for compensation.

So... I'm not justifying the practice but also business wise, it makes sense that they try to keep control of everything around their product.