> I will never understand why this is so rage-inducing for people
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.
Driving in Melbourne -- I won't generalise to the wider Australia -- is often much the same.
Comparing to the experience of being a passenger on a bike in Saigon, the level of cooperation there is way higher, possibly due to a sort of necessity. I had this feeling while observing traffic there, that, while the latency and throughput of the roads during high traffic times are still kinda awful, at least for bikes there's a slow continuous progress that simply wouldn't exist without cooperation.
Funnily enough, my experience driving in Los Angeles was distinctly not terrible. Traffic was usually good enough to drive in, though there was very little regard for the speed limit on the freeways! I suspect I may have just been lucky to miss the worst of the traffic.