Interstate etiquette

As if there were such a thing.

Why is it that a person who wouldn't think of ever cutting in front of someone in the checkout line at the grocery store will do just that when driving in their car.

When there is a line of cars in the fast lane passing a slow-moving truck and someone passes everyone on the right and cuts in front of someone, that is just as inconsiderate as cutting in line elsewhere.

Why do they do it?

Lack of repercussions. In the grocery store, they would have to actually deal, face to face, with the person they cut in front of. In their car, they're anonymous.