I think part of the reason is because it’s fairly anonymous. You can be angry and let out your hostility and feel free to do what you want. You might not swear at the people at work when you’re frustrated, but in the car? That fucking asshole cut you off! You can just scream at him and flip him off and go in your merry way.
Also, a bunch of people do not know a lot of the road rules. My sister won’t be in a car when a particular friend is driving, because that friend is always angry at other drivers for not giving the right of way and other complaints and her girlfriend is the one who is wrong. It drives my sister crazy and she finds it very unsafe.
Then there are people who think they are great drivers, but wind up being reckess in the end, because they don’t take into account not everyone on the road has the capabilities of race car drivers. My husband instructs racers, he knows the capabilities of cars better than the average driver. If he drives on the road like he does on the track he runs the risk of all those people who don’t feel comfortable driving fast, who can’t predict his next move, who don’t have the reaction time a race car driver has, causing a crash.