Bear in mind, I have no idea how wholesalers work in this day and age. But once upon a time, a wholesaler like Costco sold stuff at cost (e.g. they charge what they paid the supplier for the product), and their revenue derived from the annual membership fees. So it's not in Costco's interest to start selling Kirkland Brand Extra Virgin Olive Oil at Safeway, because it wasn't a profit center to begin with. The whole point of selling their own branded product was:

a) To reduce costs by controlling the entire means-of-production

b) To ensure quality control on the product and build faith in the brand, which in turn

c) Pulls people into the store, forcing them to buy a Costco membership if they want the product.