Totally agree, but the US is huge!
It would be much more informational to take into account things like average number of cars traveling between various locations, number of lanes between said locations, residential density...
Another issue is mobility in America. It's not uncommon for people to move across the country for work. So cities like Portland or Austin don't expect the sudden boom in population and increased traffic that entails.
Meanwhile, the leftmost lane attracts people wanting to drive faster than traffic, the people who just want to get one car ahead, as well as the people who just want to drive without having to let cars in all the time.
If there's only two lanes -- like on most rural interstates -- and sufficient volume exists to prevent the left lane from actually clearing out cars, this creates a tragedy-of-commons situation in the left lane where there's more empty space in the right lane than in the left. A subset of people will then use the empty slots in the right lane to try to get ahead, making the situation even riskier.
But this poster's right; the root cause is not enough lanes to ensure a high level-of-service.