There's a couple of factors that go into people buying these games. I'm assuming you're talking about Madden games in general and not just the DS versions:
1. Most people don't actually buy each one year after year. Some people, like me, simply buy one every few years to update the rosters and gameplay periodically. Of course this purchasing strategy doesn't quite show up on game sale charts.
2. Fans of the sport will honestly, truthfully, care about the roster updates. That may be a foreign concept to you, but having your favorite team's newest draftees and signees on the squad is actually a major selling point. The whole idea of sports simulators is to SIMULATE THE GAME, and you can't really do that when you don't have all the parts (players) to do that.
3. While it may look like the game doesn't change year to year, there are still gameplay and mode changes that happen. They may not be much, but little stuff like new animations, new running mechanics, and new game modes, will get the Madden games that much closer to the real sport, which is really what all of us want.
Then again, if you are talking about just the DS versions, then all I have to say is that yes it is the same crap year after year and I don't really understand why people buy them. But if you still don't understand the lure of sports games then all I have to say is that yes, people do get off on managing salary caps and getting a sweet block on a blitzing linebacker. You may not ever understand nor do I expect you to, but just know that there is a huge market for these games in the millions of people that watch and ENJOY football every week.
--- "The face of a child can say it all, especially the mouth part of the face. - Jack Handey http://i512.photobucket.com/albums/t329/JollyWaffle/kermit.jpg
How come EA makes these games EVERY SINGLE YEAR, and ppl actually BUY them??
Lack of competition. The games produced by EA aren't anywhere near as bad some people make them out to be. They're actually well-made and reasonably polished if imperfect products.
That said, I only buy a new EA sports title if there have been significant or notable improvements made over the version I currently own. I recently picked up a used copy of this particular game because it was cheap and I've been itching for a portable football game with more depth than Tecmo Bowl Kickoff. Madden 09 DS is pretty good, but I'm hoping EA will reboot Madden on the DS in 2011 and give us the innovative football sim the DS deserves.