True, but they still have to add tons of art, animations, and graphics every year. They also MUST come up with a gimmicky feature for marketability. Sadly, the really casual audience isn't going to pick up a copy if there are no "awesome new features!" to read about on the back of the box. There aren't enough sim style gamers to just market it towards us. Then finally, they have to improve gameplay, dynasty, etc. Often when they mess with this stuff, it breaks other stuff, which results in having to spend time fixing it.
Even though they certainly aren't starting from scratch every year, it is still amazing that they get a game out in 9 months. Yes, 9 months. They can't do anything for a month and half before the game's release because they can't risk causing massive damage to the game that they wouldn't be able to fix in time. For a month and a half after the game's release, they have to perform maintainence. And don't say, "It should be perfect at release." Not a single piece of software EVER released is completely bug free.