Except unlike prisoner's dilemma, it's not a zero (or negative) sum game. The entire traffic flow can speed up if zipper merging is done. The "late mergers" don't have to risk losing their game of "chicken" while the "vigilantes" don't have to deal with the reckless maneuvers of some of the former.
People don't do zipper merging because people are human and drive emotionally [1]. The stereotypes of the conservative early merger claiming the moral high ground and the reckless late merger doing anything to save a few minutes are the cultural results of this mentality.
[1] Another example is the special breed of driver who stops the traffic flow on a major road (which has the right of way) to let someone on a minor cross road or parking lot exit onto the major road, thus inconveniencing everyone behind them for the self satisfaction of having helped the one person they can see.