C'mon... you know that's not true. Maybe it's a regional thing or a geographic thing but I've lived in multiple US cities on both coasts and in the north and south and have had people use the term on many occasions. I even had a friend when I was growing up whose parents designated the role of "co-pilot" to the eldest when they were old enough to sit in the front seat.

"I need you to be my co-pilot and watch out for cops."

In my experience, you're correct. "Shotgun", "copilot" and "navigator" are all terms used interchangeably in my family. And urban dictionary has entry for 'co-pilot' from 2005 referring to the shotgun passenger.