>>3625The trick is membership stores are explicitly (that is they say the words verbatim out loud) trying to invoke the sunk cost fallacy.
So by charging X$ for memberships they're trying to trigger people into trying to get their money's worth. Lots of foot traffic to and into the store. Once people walk into the store for eggs or milk they're bombarded with in store advertising and while most people probably won't walk by a 2000 dollar play-structure for the kids and buy it, you might do something like getting a dozen muffins you didn't plan to buy that trip. Or a chair. A comfy blanky. etc. Even the most stalwart autist isn't immune to ads and impulse buys.
Personally once I got kicked off my mom's membership I started buying a couple gift cards a year. For the price of floating Costco 75 bucks a year no interest I could make 3 trips into the store with 25 dollar gift cards.