David Friedman se faz esta pergunta. Vou citar um trecho, mas leia tudo.
Why do we tip waiters, taxi drivers, and others? Why is tipping a common custom in some societies but not in others?
The obvious economic explanation is that tipping provides a way in which the customer’s information on quality of service can be used to reward good service and penalize poor. That explains why tipping is not limited to unusually good service; if the tip for average service is zero, the customer cannot reduce it further to punish unusually poor service.
This explains why we should tip but not why we do. A regular customer at a restaurant may regard the benefit as a private good, an incentive not to provide good service but to provide good service to him. But that does not apply to the passenger in a taxi, who is unlikely to ever see the driver again, or to the traveler who eats at a different restaurant every night. Yet they too tip.
Friedman prossegue e especula sobre a diferença entre europeus e norte-americanos. Eu pergunto a você, leitor: e os latino-americanos?