Who else competes in that specific market though? Sam's Club and Smart-and-Final are the two I can think of, and it's been a while since I've seen either one of those. Oh, and actual restaurant supply stores, but those are different, imo. Costco's not directly competing with Safeway, for example, as they are different parts of the market.

Can you provide some examples of monopolies for context in this discussion?

That convenience is everything. Doing it well, and not falling into the trap of putting profit (too far) above users is the challenge that is too hard for other players (except maybe GoG) to get right. It's like WiFi. You go somewhere, connect, it works, and then you don't think about it unless it's surprisingly fast, or there are problems with it. Everyone else's offerings on this space just feel janky and liable to take your money for some reason. Steam, for the majority of its users "just works". That's not to say there are zero buys with the software and that nobody has valid complaints about it, but just that in general it's great.