@Xcheldrux because that could pretty much end up killing him. Remember that part of the reason they went to those islands was to help Naofumi recover from the after effects of using that shield's kill (Blood sacrifice). Raphtalia thought (and correctly) that in that state it would kill him.
What kind of silly billy lets medicine expire in their inventory? Obviously nobody carries rare elixirs to their respective sequels, so they evidently don't last between them. It's effectively like hoarding soda bottles at that point.