Do you really have to over simplify it I'm sure they have missions that have there own challenges when you think about it all games are simple do you wonder how fast a person will get bored of a game where u must drive a car along a road really fast in a race against other cars, Do you think people will get bored quickly of a game where u run around killing aliens with futuristic weapons, do you think people will get bored quickly with playing sports. Because if you do you would have to wonder how any game ever made would be successful.
Sorry just my rant for the day.