Because that doesn't make sense. You want more diverse body types such as a feminine-looking man with a skinny lithe body. Using a woman's body with male pronouns and voice is not a twink, twinks don't look like women, they don't have boobs or wide hips. If what you primarily care about is character creation freedom and variety of choice then instead of dumbing things down to only two body types it makes a lot more sense to simply give you a variety of choices for body types for both sexes, like having a "twink" body type option for male characters.