That's a fair point, but they're released around the same time to compete with each other and generate cash for the company.
Let's say, for agument's sake, that Bioshock and GTA V are released on the same day, and that people buying those games (and this is not a totally unrealistic proposition given the state of the economy), only have the money to buy one or the other. Whichever way you look at it, such as the public splitting 50/50 and generating a million sales, that's still only a million sales.
Now if they're a couple of months apart, people may be more inclined to buy both as they can have the money handy. Releasing two top titles so close together makes no sense from a business point of view.