My health insurance--various over the decades--has never covered mom & pop pharmacies, only big chains, just like not every doctor is in-network...

This might be a mis-perception on your part (depending upon what you mean by "over the decades") The large scale cost-cutting and low price bidding strategies of both pharmacies and health care providers is something of a comparatively recent phenom. If you were prescribed a "very expensive antibiotic" as a dispense as written presciption, it's highly unlikely that you could've gotten it at Sam's Club or wherever for $10 for a 90-day supply.