Wow, this thread got really interesting all of a sudden.
I'm starting to understand the sport video game phenomenon. Basically, it's not the game itself that is what keeps millions of people coming back to franchises like Madden. Instead, it's that it COMPLEMENTS your existing enjoyment of the sport. Madden games work like a personal action-figure version of your favorite sports team. You can simulate what your favorite team will do this season and beyond.
People get it every year because the sport that they already follow has changed through rosters, rookies, and players' ages and abilities. I still don't buy the game every year, but I now understand why people do.
Another interesting suggestion I've gathered from this thread is that the time apart from a game series helps keep it fresh longer. For example, I recently picked up all the GBA Castlevania games and have been playing through them all in order. Needless to say, I am agreeing that the franchise is getting stale. However, if I played one GBA Castlevania a year, I wouldn't feel so bored by the overwhelming task of playing EVERY GBA CASTLEVANIA + THE DS ONES SO I AM ALL CAUGHT UP FINALLY AND CAN FEEL LIKE A MAN NOW.
As a current example, I have been working through fully completing Mass Effect 1 so I can play the 2nd one, but I would probably enjoy 2 better if I took a break from the Mass Effect universe in between.