They don't make as good games.
In regards to sales only being on games older than one year, you're wrong there. Both Dragon Age and L4D2 were 25% off under a month after release, which means you would have been better off waiting a few weeks after release before buying the game, which is what i'm trying to get at.
What the sales do is persuade people who were most likely never going to buy the game to buy it because they feel they shouldn't miss out on a good deal.