I don't know the answer but I wonder the same thing.
One intriguing view of it is: "... most popular pharmaceuticals, NSAIDs, statins, anti-depressants, anti-diabetics, etc., also have substantial antibiotic activity. Most of these pharmaceuticals started out as phytoalexins and then were found to also have pharmaceutical activity. Pharmaceuticals are just repurposed natural antibiotics. When you take an aspirin or Metformin or a statin, you are taking an antibiotic. When you take a pharmaceutical, you are selecting for multiple antibiotic resistance plasmids in your gut flora and you may be making the next superbug."
