I might going to say something outrageous, offensive or out of reality, but fuck you.
Then every Megaman after the first should be boring and annoying. They follow the same idea (intro stage > chose boss stage > defeat boss > repeat), the concept is the same (you die in stages to lost your last life against a unfair boss fight) and the gameplay is the same (jump and shoot man). But why is still fun to play Megaman?
Basically because Megaman is a game about challenging stages and unfair bosses (unfair if you don't get his weakness and you are with low hp), the game keeps this core and provide "unique" challenges in each new stage. It doesn't mater that the game mechanic is the same or that the concept is just a repetition. What matters is that in each new stage you feel a new and different challenge, one where you must get better to accomplish.
In counter part, why in AC this doesn't happens? Basically because the game doesn't provide a "new challenge", quiet the opposite, the game provide the same challenges with the same mechanic and concept. The challenge in AC is climbing walls and jumping, which are mapped, and you don't have to think too much on those. When there is a enemy, you already know if he is strong, weak, agil, etc, the game doesn't hides anything to you. And the game is using the story as a excuse to keep making the same game without real changes.
AC, Uncharted, God of War, and others, are the examples of deviated games. They are trying to hard to be something between games and movies, but they forget their are game first.