Because you perceive your car as your own private space. But in traffic, others constantly “invade” that privacy and force you to react to them.
Additionally, going at least the speed limit is perceived as the “normal” mode in a car. When someone in front of you forces you to stop, it feels like they’re trapping you, cause you actually want to move, but can’t.

Basically, our lizard brains aren’t wired right for car traffic, and some people can’t deal with that.