It really isn't. There's no good excuse for anywhere in that region to have been untouched, nevermind that there are more than enough people who know that "apothecary" is a synonym for "pharmacy." It's not that archaic seeing as how, you know, it's still in use. By businesses that deal in that sort of thing even... And since we know there's at least three well-organized groups scavanging the region, it's even less probable than if it was just Rick's group.
I should have said "a good excuse for a handwave", which is generally how things work on this show. And that's more excuse than the gasoline has.
I don't give the Wolves credit for enough book smarts, and I think the Saviors are far enough away not to count. So the only question is why the Alexandrians never thought of this place before. And a good excuse for a handwave is: it never occurred to them to look under "A" when looking for pharmacies or medical supply stores. Yeah, I know. Work with me here.
(That's assuming they all know the meaning of the word. I'm sure some of them did. Porchdick Pete must have.)
More realistically, there have probably been better places to search up to now. I think we're just now seeing the scouring of the last few untouched places. What will become unrealistic is if later they stumble upon yet another treasure trove of untouched supplies, like a "Wal-Mart" that no one remembered about. Sooner or later things like that must all dry up.
It would be great if we could see people redeveloping crafts and skills. Hilltop has the right idea. Making new bullets is one thing, but there is no support for all the modern paraphernalia Alexandria uses. And they don't have the resources to develop more. Plastics, microchips, refined petroleum, etc. are out of their reach no matter what. But if they start learning how to forge, smelt, build steam engines, extrude copper wire, isolate chemicals, blow glass for light bulbs and vacuum tubes, etc, then by the time the last solar panel goes down they could comfortably sustain themselves at an early 20th-century level (with the potential to keep advancing).
If they keep operating the way they are, then eventually they'll just run out of tools and find themselves dumped at the beginning of the 18th century. Without the prospect of advancing any time soon.
Would old-style black powder even work in modern firearms?