Zipper merging is ideal in certain situations but not all of them. It reduces the total “length” of traffic the choke point, but doesn’t necessarily increase the total throughout. It’s primarily meant to prevent traffic from backing up to other lights and intersections on the road. The throughput optimum for lighter traffic is to merge earlier (though not miles back) to maintain speed, and people who force their way in at the last second cause the standard “traffic wave” problems. That’s why it isn’t quite as clear cut as people make it - the optimum behavior is situational and that level of complexity is not well gasped by your average person who is profoundly uncurious of the world around them.

Then there’s the issue of people wanting to zipper merge in places where it is completely inappropriate because it blocks a travel lane. This was a weekly debate on our local subreddit when I was still using it. Enough people insisted that a highway interchange should be treated as a zipper, despite that blocking the main road, is high enough that I am convinced that the zipper narrative has been a net negative. Though I also concede that most of these people would still be idiots about it without the plausible justification for bad behavior.