I get what you mean. You'd rather play the same game, with a different coat of paint, every 3 years.
But it is, to a degree. Series and franchises achieve success based on constants, not changing every itteration. The reason people like those series are those constants, not the changes. FF had some great features each itteration, the ATB and menu combat, similar names for things (magic, summons, characters) yet different world and story. They are what makes FF games unique else what's to say the last Star Ocean games wasn't a FF games (other than the title)? How these constants are changed and differ between each itteration is the key (each new game has a different magic system within the ATB constant).
It is the basic constants in gameplay that many people want, it is what attracted them to the series to begin with. Tekken is still a 3d fighter with a button with each limb each time, each CoD game is a FPS with aim down sight and quick gameplay, these are the basic constants in gameplay that make the games unique yet, the games have adapted over the years (Tekken has walls and is far quicker, CoD is more sci-fi now not WW2).