The most important is that airlines have the ability and motivation to force passengers to accept unfavorable terms, and passengers have no ability to negotiate more favorable terms. Many routes are only served by one airline, so there isn't even competitive pressure in those cases. There's also a financial incentive for airlines to mistreat customers, e.g. by overbooking flights, canceling underbooked flights, and delaying non-mandatory maintenance until the cheapest or most convenient time.