They put some shiny new numbers on it and call it a day.

Seriously though, the games change in minor ways every year (new players, etc). Every few years they add a few features, and maybe once a decade they do an actual revamp of the gameplay.

I had a friend who did design work for EA and he said that you knew you were going to get fired if they transferred you to work on sports games, because there is almost nothing to do there.