It's because we already have these Final Fantasy characters in games known as Final Fantasy. When creating a game, it's up to you whether or not to go gung-ho original, and old material can be cool if applied with skill, but I think that playing a new game and finding Vaan or Vivi with a new look but pretty much the same motivation, demeanour, and complexities (the thing about archetypal characters is that even when they're complex, you can guess what makes them so with decent accuracy -- the adventurous devil-may-care type will be searching for secrets from his early childhood or trying to forget the catastrophe that befell his family, the timid type will always feel like he doesn't belong because of his amazing powers, etc. (If you have original stuff to place in this department, it would be a massive step to the good and I think I would forgive you almost entirely for reading out of FF's notebook)) would make things a lot less interesting.
Then again, if you can actually put an original spin on the archetypes that isn't amazingly predictable (as most tend to be,) that'll be another story. So long as the end result is a character who interests the player and doesn't appear to be a horribly obvious RPG archetype, I don't really care where you got the first draft from.
