For me it's got alot to do with the characters and plots. I think we can all have a general run-down of how it plays out: Generic optimistic youth is somehow the last hope for a world/universe/kingdom/etc, and has to go on a quest to save said world/universe/kingdom/etc. Over the course of the game, he has to gather an assortment of characters usually consisting of: Female childhood friend who wants to get in his pants, serious foil/rival to main character, Spunky female who's only in the game for sex appeal, The healer, and normally one other character who is heavily linked to the main plot.
There are sometimes some differences, and naturally there are exceptions to this, but alot of it in my opinion comes from character archtypes
I honestly think if the genre actually focused more on giving us some variety among protagonists, we could have some great games from the genre. Hell, this is the main reason i loved Drakengard so much is because it turned all this on it's head: Your main character was a complete psychotic murderer who just so happened to have a grudge to settle with the big bads, meaining saving the world was VERY much a secondary motivation.
I just want them to do something different for once: Give us an older protagonist (Nier did this, and it did it very well, hence why i wanna see it again), hell give us an outright bastard for a main character. Just do SOMETHING new!
What i would LOVE to see is a main character more along the lines of trying to do what Griffith in some aspects: Mainly, an extremely charismatic and skilled leader who has to save the world and decides to do this by rising through the ranks to the top of nobility...and are prepared to do any damn thing they have to in order to reach it: The party members they gather? Just tools to get what they want (Bonus points if they literally view them only as a role eg. think of the tank as 'the one with the big sword'), so they'll smile and manipulate them so they get what they want. The side-quests? Do it to get a reputation around, that's it. After all, a hero can't get very far without a reputation now can they? Make them SEEM nice and like a proper hero at first glance, but after seeing their thoughts and opinions, you'll realize they are an utter bastard who only cares about them self. (You are allowed to take this Square Enix, i am literally GIVING you a new main character archtype to experiment with!)
So yeah: It's the characters mainly being recycled a bit to glaringly for my liking and their apprehensiveness to actually doing anything NEW with them.