It honestly baffles me from a money making standpoint, but I'm obviously no high level executive. BUT, if you think about it, the amount of people that will buy the game will decrease with the next game in a sequel. For example, if someone buys the first XIII, and doesn't like it, or doesn't like it enough to buy the sequel, it's only natural that less people will buy the next. It's just like when you see a series on youtube, the first episode always has the most viewers, then it declines with the next and next.
But, there could also be less cost in production since the whole game has already been developed from the standpoint of characters and whatnot. So maybe it balances out.