I view origin stories the same as say....legend of zelda, metal gear solid, assassin's creed, mario brothers, sonic the hedgehog, quake....that is not your character, it was handed to you. That story is not one you made up, it was crafted and put into the game with game mechanics supporting it.
If anything custom characters are flat because there is no story at all for that character beyond a generic game story, so you are missing out on a quarter of the game by not having one of your four characters have a built in back story with possible rewards or lore tied into the game.
I did not play a lot of D&D growing up, but I distinctly remember there used to be pre-built character cards and minor stories that can be made by the DM handed out to players. If anything, the pinnacle of roleplaying is playing someone you didn't make up and having to adapt to a role provided instead of just making someone similar to your own mind set, but with elf ears, or mimicking a cool story I read in a book somewhere, or even better, making a generically evil character who "mwa ha ha's" and does all the evil things with no plot because it is fun.