On the one hand, I'm aware that at least 5 million people in the US do not have a valid government ID. On the other hand, traveling within the US requires you to show a piece of ID wherever you go - even when taking a Greyhound.
So let's say you're an American who has a large number of cash in their pocket but no ID whatsoever. Would you be able to get from New York to San Francisco or from Seattle to Miami? If so, what are your options? Even though I do have plenty of identification documents, I find it enjoyable to be able to travel anonymously.