I completely agree that it would be better if it was that way and that a genre if this scale should have it.
Just an FYI that is not a small ask.
I used to work in video games. I specialized in making customizable player characters. The amount of resources to go from swappable geometry (the way it is now) to swappable geometry + blendshapes to tweak features is massive.
There are some tools that you can implement to automate some of the work, bit there is a huge amount of work that needs to be done to make each body/face tweaksble. Plus you need to make all of the weather items like hair, hats, clothes, work with each of the steps of the tweakability. So nothing the player can do can make the geometry interact in undesirable ways. Like the body protruding through the shirt or the nose through the helmet.
Lots of tricks to make it work, but it's still a lot of work.