Actually I wonder the same thing about halo. I liked the first Halo then Halo 2 and 3 turned into multiplayer fest. But still, Halo 3 has a different story. Not everyone buys it for online. Im going to burrow halo 3 to play campaign and thats it.
Zelda has a different story in every game, and they add alot of new things.
Final Fantasy has new stories on each one does it now? New spells/skills? I dont play FF so I dont really know, but I doubt its the same.
RE has all different stories and new things. Especially RE4...