The market for computer and video games gets more and more saturated. Because of that there is no question that a game must have both: attractive visuals and attractive game-play. Photo-realism helps with the visual part.

It can be another style for sure. But look at the sales in the last years. Good games with style and game-play (like Outcast, Beyond Good and Evil) that differ in it's style do not have best sales. Mainstream realistic looks that approximates photo-realism sell better.