But it's an explanation nevertheless that fits in the world provided.(Considering the parasites and what happens to her at the hospital.) It's not a copped out super power like 'Oh she needs to see the sun constantly, that's why', but there is reason for it.
I understand the "within the world created" comment, but you DO realize this is a fictional creation by a person, right? EVERYTHING about this game is entirely contingent, all it would take is for Kojima to have said "she can have a flak jacket, at least" and nobody would have been the wiser. Do you understand that saying "that's just how it is" is not a justification for WHY it was made to be that way?