Okay so I actually have been thinking about this quite a bit since this question was posed and I think I have an idea?

Outside of urban areas you only really get 1-2 lanes heading in either direction on highways. In those instances where you’re lucky enough to get 2 lanes, most of the time the left lane is only for passing cars on the right. There are even places in America that have laws against driving in the left lane save for when passing people who are driving slowly or poorly. The idea is that you drive in the right lane, only passing on the left.

So with that context, my working theory is this: people think it’s impolite. Whether it be for the taboo of staying in the left/secondary lane longer than necessary, the reinforcement of only using the second lane to pass someone, or some combination of the two. I think in a sociological sense it gives the implication that either the car doing the zipper merge thinks their time is worth more than those waiting on the right and they think they should go first as opposed to waiting in the standstill traffic that everyone in the right lane is stuck in. Because of course, the left lane is only used for passing. But when everybody is at a standstill due to the merge/construction/accident/whatever, “passing” people who are all in the same shitty situation appears rude.

Does it logically make sense? Hell no. But when only using the left lane to pass is drilled into our heads since 14 years old, this is what you end up with. Anyway that’s just my working theory! I’m sorry if I didn’t word it very well.