It is true that the best pet stores are specialized pet stores. I have done some reading on store reviews a few years ago. The best reviews I have ever seen was a fish store specialized in Cichlids (a type of fish). They truly know their stuff as the customers put it and they were extremely into the hobby themselves.
I totally agree there are very few people who can know every pet species or every pet product. Even less likely one of these "all knowing" people would be running a pet store. 
The store policies are set by the upper management from corporal level. I do not know why they set such policies, but I am sure they somehow determined it is the best way for more profit.
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