Some people don't, Some people do. Nobody said you were required to like everything about smash. Just look at tourney players. However, Sakurai wanted to make a full solid game, and this DOES include a "suppously" good storyline with it. Also, Maybe he didn't feel like making the same adventure mode 3 times, where you always fight masterhand, the wireframes, and some other random players. Considering adventure mode has CO-OP, I see no problem with teaming up to beat the story.
Some people dislike the music, some people dislike the items, the stages, the characters, but that isn't going to stop them from improving upon it, and the adventure mode is one of them.
Thank you.