One thing not mentioned in many of the comments is that Costco also rotates inventory based on season. A huge percentage of the inventory will change depending

One thing not mentioned in many of the comments is that Costco also rotates inventory based on season. A huge percentage of the inventory will change depending on what time of year it is and that can lead to both a kind of scarcity buying ("who knows when they'll have these pants again?") and novelty ("I wonder what they have this week?"). That, plus the bulk savings, plus the membership benefits (an almost absurd return policy), plus a well curated inventory of mid-to-high quality items (plus savings of own-brand "Kirkland signature") means Costco shoppers buy a lot, they buy often, and they keep doing so for years and years and years. It's highly fragile in the sense that it's so simple, but other competitors haven't made a huge dent in the same segment (Sam's Club, BJ's, etc.). Costco is doing some magic combination right and they'll be the study stuff of b-school grads for decades. edit one other thing. Model #s for Costco are often unique to Costco. It also makes comparison shopping difficult. Try comparing a printer or TV model # at Costco to others on-line. Chances are it only exists at Costco, which means both exclusivity as well as an assumption of quality + price due to other in-store contextual buying. People think they're buying something that's both special and a good buy, which further validates their yearly membership dues.