I have a pet theory on why tipping remains popular in America. It goes something like this: Americans want the illusion of choice, even if they repeatedly make the same predictable choices. So, even if most people just add 15-20% (whatever the local norm is), they, in theory, have the ability to pay more or less, depending on the service. If the tip was factored into the price, the customer would lose that ability, even if they don’t actually use it very often, if at all.