Because Costco are taking a risk. You can only have X different widgets on a shelf, its a limited resource so you can't just let every brand in to sell their wares and take a cut. This in turn means Costco has to take on other risks. They're liable for fakes, they're liable if something doesn't sell (I assume Costco probably does sale or return for new suppliers? But there are limits to this). They're doing the fulfilled by Amazon role as well.

Its the difference between a Farmers Market and wholefoods. You as a consumer probably have nothing to do with the market organisers, and they don't have any particular responsibility toward you.