Putting my armchair psychoanalyst hat on: I think American society embeds a need to be the "winner", and are you winning if end up behind another driver who's contending for "your" spot?
If you've driven elsewhere for a while, you start noticing subtle driver differences, such as drivers who want to merge into your (slower) lane never braking to merge behind you and always accelerating to do so, even when you're at the tail end of a vehicle chain in your lane.