So in the USA people automatically have both a passport and an ID?
Cause it sounds the same as traveling between EU members, but here most people don't have both documents. They either choose a passport or an ID card, cause these documents cost quite a lot of money where I live. I chose to buy a passport, so that's my only form of ID. It's not required to travel to Germany for example, but I just don't have anything else.
For domestic flight, the requirement is govt issued ID, so it can be a US driver License, passport, Nexus, or just a State ID if you don't have driver license. Not a lot of ppl has passport, just until recently, you don't even need a passport to cross the border to Canada, which is probably the most often visited foreign county for Americans as you can imagine. But of course it has changed now that a passport is required to go to, and to return.