Costco really isn't a retailer in the traditional sense. If you look at how they operate, aside from the fact that they allow the public to buy things, they really operate as a distribution center. From where they build to what they carry, it's not optimized to get the most customers because they know they are profitable from membership fees alone.
Because of that, I think its silly to compare Costco to any other retailer in any sense. If you never even walk through the door they've made money on you. That strips away the need for a lot of things that other retailers need to do to get you in the door.
They don't care if you get in and out fast for one item, their average cart value is really high. If you know you'll need to wait anyway, you'll do all your shopping there in a large trip and there is less "between order" time spent for their cashiers (who aren't really cashiers but basically warehouse packers, and you're the picker).
This extends to everything they do:
- They don't need to carry less profitable products to get you in the door, they optimize for least products and most profit
- They pay more because these aren't store employees but a combination of store and distribution center employee. This is supported by "market salary" a bit if you compare walmart distribution center employees you'll see they're actually higher paid starting salary, Costco employees fall on the upper end of the area between walmart store employees and walmart distribution center employees
- They won't build places they can't get people to pay that membership fee. The membership fee is a bit of a means test if you think about it. Poorer people can't afford to buy in bulk even if it saves them money, if you can't shell out the money for the right to shop at their store, then your average cart receipt doesn't fit in with what they're looking for. They'd probably be able to build profitable stores in a lot of places, but the membership fee structure means less stores but very low risk. You know if your market can afford to pay for somewhere to shop, they can afford what you're carrying. And it doesn't matter at that point because you're already profitable anyway.
They really aren't a revenue optimized company, they don't care about a few missed sales that you run to Acme instead of Costco because you only need one thing and don't want to wait. That's exactly what they want - a nice big cart every time you come.