They are fun, no matter how unoriginal they are *looks at this game* However, some, like Mario world and SMB3 are amazing for replaying, but some (like NSMB and NSMB2) are just there to beat and they later take dust.
"Behold my powers! I am the most powerful Pokemon in the world!" -Mewtwo 3DS FC: 4339-2716-5379
For the same reason people play any sequel. The older games, no matter how good they are, get stale after countless replays. Why did they even bother making Galaxy 2? After all, we could just play Galaxy over and over again.
Ha, casual gamers. Many Mario fans, including myself, have been playing Mario since the 80's. Yes, a lot of it is about nostalgia, but I don't consider myself a casual gamer, and I know many other "hardcore" gamers that enjoy this game.
But then again, if you don't enjoy most Mario games you will not enjoy this one.