I replay games if they have an open world. For example, I beat Grand Theft Auto 4. But it's an open world, so I could still roam around and cause trouble and steal cars, even though the main story has been resolved. There are also really simple games that are fun to replay, like Beautiful Katamari.
Sometimes I just go back and see if I can beat my previous time and roll a bigger katamari.
As another poster said, games are too expensive to just play once.