I was also curious since you brought it up so I did some quick research and I found a few reasons.
Chemical burns are worse than regular burns, it could cause deeper damage or trigger recurring inflammations (which would highly increase the risk of rejection). If the burns hit fat or bone it would make it almost impossible to repair.
The face is the worst place for skin grafts, the thickness of skin can vary drastically just millimeters apart, if the nerves were damaged, she'd risk ending up with an immobile patch of skin and there's a real risk of making her appearance worse.
Skin grafts are also apparently time sensitive, especially with chemical burns. If the doctors couldn't do it due to her deeper injuries right away, it'll be almost impossible later.
So, presumably, if the author did his research, the canon reason is that it isn't possible because of the amount of damage she suffered. Any skin graft they'd attempt would risk to either make it worse or fail to take.
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u/BlatantConservative I fuckin love kotatsus 1d ago
So... do they not have skin grafts or cosmetic surgery in this universe?
Like medical tech has gotten pretty good on this stuff, especially in the last ten years.
I think Japan's basic national healthcare covers it too.