I'm guessing all of the sports titles have local multiplayer (NBA 2K14, NBA LIve 14, FIFA 14) plus isn't there some dancing Kinect game? That can do up to 6 players in the same room.
My guess is that a single console can barely handle 720p and 30 FPS for one player, so it couldn't handle the goings-on between two players in a horde of zombies. Dead Rising 3 sounds like the epitome of a rushed game that fails mostly because of its timeline.
I don't know how co-op works in DR3, but if the local multiplayer co-op though had you both on the same screen (i.e. not split screen), wouldn't be trivial to add in another playable character? Anyway, yes, it does seem like the developers ran out of time to meet the launch deadline. All the reviews say the game is really fun, but suffers some technical problems (dropped framerates, etc.). Maybe they can fix those problems with a few patches down the road. Regardless, this still seems like the best exclusive game to pick up at launch. Ryse is good eye candy but boring gameplay. Forza looks awesome but racing games are not for me. I was hoping Crimson Dragon would rock, but alas, it does not. Either that or I guess the non-exclusives are pretty good, like Assassin's Creed and Call of Duty, but, obviously, none of them have local multiplayer.