Why aren't they accessible by Hospital Corpsmen? Every round there are multiple ones of these vendors that sit there, not being used, or having one or two pills taken by a doctor.

But Hospital Corpsmen, the medical role that heals people the most, who would have the most need for access to the medicine vendor does not have access by default.

It just seems a bit silly and limiting. If you join late as a Corpsman you're extremely limited in what medicine you can acquire with your points, as you need to juggle that with any other utilities. Changing the access might even mitigate the rampant looting of medbay that happens each round for the tiny amount of free meds that are afforded to the medics.