The game originally had specific backstories, personalities and appearances for the main characters, but as the story evolved, that was changed and there was no reason not to allow selection of gender and appearance during character creation (or later in the game, using a mirror in the homestead).
I assume tripling the number of cutscenes for the history was deemed unnecessary, or not feasible, and trying to do them without showing any character features and only using gender neutral terms would have stood out more than just having them be different (in appearance if not gender) than the player characters (who can change their appearance and gender).