The origin stories are that of a distinct character, their life, background, wants and needs. Like your sister or brother.
A custom character can not be shoehorned into the story of another character. just as a stranger can be shoehorned into your sister/brother's life experiences.
The writers can not write a distinctive story for every character's life outside of their race, class, background that is chosen and the need for getting the tadpole out of your head. Otherwise, it would feel like your custom character is just another origin character.
If the writers write that the custom character is a vegan. Every custom character would be a vegan in the storyline, so there is only so much that they can get explicit with the characters story.
It would hamper your imagination of your custom character and how you would roleplay your character.
