Why do so many people save their shopping to the last minute?

Every year on the night of Christmas Eve, it is impossible to drive by a store with an empty parking lot, (unless of course the store is closed, but that's beside the point.) It becomes a hassle to go anywhere because the roads are swamped. I did that one year and once was enough for me, so why do people continue to do it? The only thing I can come up with is they want to do it one day but they run out of time so they put it off for another day; but those days continue to build up, and pretty soon it's Christmas Eve, so they go. Think of how much nicer it would be on Christmas Eve if people wouldn't procrastinate. Unfortunately, people will continue to do last minute shopping and get in a huge hassle, but if they could just get it all done early they could avoid the hassle and be happy.