Rats from pet shops often are just as wonderful as rats from breeders. They are beautiful, intelligent, sweet, and unique animals that are worth being cherished and loved. Sure, they may have a greater chance of a poorer lifespan or illness than well bred rats (my experience is that their lifespans don’t tend to be that different but they spend longer suffering from chronic illnesses from a younger age). The reasons people avoid pet shop rats are nothing to do with the rats themselves being less wonderful than breeder rats.
The reasons people object to pet shop rats is entirely to do with how they’re bred. They are, without exception, sold as products – they’re just stock, same as a cage or a bag of food and they deserve more respect than that. The shops are not able to vet new owners in the same way as breeders can, they can’t build a relationship and offer the same sort of impartial advice. There is no way for the new owner to contact the breeder, to pass back health and temperament information, so we know the breeder is not breeding to improve health and temperament (they are breeding for profit, no other reason). The staff are likely to recommend particular products like housing, food, etc based on what they sell – they are a business after all – rather than the more impartial advice that breeders can give.
These reasons and more are why buying rats sold in pet shops are bad, even before we look at the conditions in which the rats are bred for pet shops. Even if they were kept in palaces in the breeding farms, it wouldn’t fix the fundamental issues with having a profit-making enterprise acting as a middleman between buyer and breeder. It can’t be done ethically.