With plastic bags, I can get the entire grocery load into the house in one trip. On average, that's 10 bags. Try that with paper.

Now, I grant that paper got better once stores started putting handles on them. However, they seem to rip nearly half the time. And when paper rips...there goes the groceries. Plastic gracefully stretches and you can at least get the load to a safer height to minimize loss.

Or maybe I have an inordinate amount of grocery accidents.