I don't remember the last time I bought an FPS for the campaign mode?
I agree they could be more fun - but it doesn't really bother me.
I have bigger issues with the games that have a story line and the boring side quests, cause it just gives the makers an excuse make a very short story line and fill it with boring side quests that nobody wants to do - even more so on PC where nobody even cares for the achievements.
For example, I completed Assassins Creed Brotherhood AND Revelations in two days total...
It actually took me longer to download the games than it did to complete them, and all because I cba to go looking for feathers or whatever you have to get.
Another one was Batman Arkham City. Having completed the storyline, the % completion on the start menu was 24%... How can makers really make a game where 76% of it is collecting Riddler trophies or rescuing one guy from getting beaten up by thugs - which doesn't actually give you any benefit at all...
In the days of Zelda, at least doing all the side missions gave you something substantial like a new sword or more health so you could defeat the final boss easier...
GTA is about the only modern example I can name, where I actually enjoyed playing all the game, and it lasts a long long time (despite occasional repetitiveness of missions) without even doing the side quests. I'm looking forward to GTAV!