Heh, a lot of people really liked FF13, though. It's a game people either love it or hate it.
As for the series name, I think it's to establish a "brand" name. Final Fantasy, Grandia, Wild Arms, are some RPG series where every game is its own thing. I think Breath of Fire is another one. No direct sequels but maybe a couple things similar like mascots or whatever. Like D-Squall said about Final Fantasy, Grandia 2 has nothing to do with the first Grandia game. And plenty of gamers disliked Grandia but loved Grandia 2.