Because they have no choice. There are currently 2 aircraft companies in the world that make commercial jets: Boeing and Airbus (we'll ignore Comac for now).

Because they have no choice. There are currently 2 aircraft companies in the world that make commercial jets: Boeing and Airbus (we'll ignore Comac for now). If Airbus somehow bought up all of Boeing's manufacturing facilities and capability, leaving Boeing as just a re-badger, then airlines would have no choice but to buy an Airbus that either has an Airbus badge or a Boeing badge.