Not every game works or needs to have a co-op campaign, those wishing to bring a buddy along for Ghostbusting can use the multiplayer part of the game.
Actually, I'm a firm believer that every game should because as somebody who has a very strong relationship with his brother I find it pretty annoying when I can't play games with him.
Ok, but a strong relationship with your brothers means nothing in this scenario. I have a strong relationship with friends, but that doesn't mean I wish all games to be two player. Just because you believe all games should be two player, doesn't mean it should be that way. There are a lot of games that are designed to be single player. Trying to add another player means they'll have to rethink things to accommodate a second player, potentially effecting the game. A lot of first person shooters are meant to be single player.
Grandtheft Auto? How are you gonna make the story a two player co-op game?
Final Fantasy or any RPG game? And I'm not talking about multiplayer online games, any RPG where the story is focused on one characters journey.
What about Resident Evil? It's a solo survival story. Granted there is a usually another character in that game, but they venture off on their own and meet with your character during certain events, so adding them a second player option won't work. Also, ammo in the game is scarce, does that mean they have to add more so the second player can get more? Way too many variables here.
Not everything can be two player. It doesn't matter if you believe it should, some games were just not designed to be that way.