(Edit: I'm rather displeased that the comment I'm replying to here is getting downvoted. It's in no way deserved. He asked a reasonable question as a European t

(Edit: I'm rather displeased that the comment I'm replying to here is getting downvoted. It's in no way deserved. He asked a reasonable question as a European to whom this aspect of American culture is, not surprisingly, alien.) It's an embedded cultural assumption that gets written down for people who abdicate thought to what their computer screen literally says. Historically, almost everybody in the US has a driver's license, and almost no one has any other meaningful form of ID. Many, if not most Americans will never leave the country, making a passport useless, and up until a few years ago, a US citizen could travel throughout much of North America and the Caribbean without one anyway. They say it because it's what people say. Treat it as an historical quirk of American English.