Similar to buying a bike at Walmart vs buying a bike at a bike shop. Walmart does not hire bike mechanics, their bikes are of poor quality with low grade parts, often incorrectly assembled, and can offer little if any assistance when a tune up or repair is needed. Sure, they may have employees that have some mechanical knowledge of bicycles, but that is not their top priority when hiring folks.
To make a broad generalization, Petco/Petsmart/PetSuppliesPlus/etc are basically Walmart in this analogy. A reputable breeder/reptile specialist pet store will be far better equipped to offer tips on husbandry and also intuitively be able to assess the health and well being of the animals they are selling.