Because it's supposed to be a single player experience? As much fun as Co op is, not every game needs it. Just like not every game needs competitive multiplayer. There are dedicated co op games. This is not one.
This game was never going to have co op. Why is it being picked on so much more than any other game that doesn't have co op? They never said anything about co op. Why are people acting like they were lied to and are disappointed? Why aren't you complaining that any other game doesn't have co op? This whole thing is really stupid.