>>513230627 (OP)
If you shop there, you earn credit for every dollar you spend and you can even receive checks from Costco in the mail, or you can ear fuel points to spend at Costco gas stations, so in reality, you save a lot more money by shopping at Costco than you would have saved by hunting bargains at ten other stores across town.
Hunting bargains used to be a good strategy when gas was cheaper and it didn't cost as much to drive all over town to save a buck here and a buck there. Also, the items that are cheaper at some other store are only going to be cheaper there for a limited time. The prices fluctuate so that they can make their money back on other items while one item is discounted, and then make even more on the same item two weeks later when the price goes back up again. If you get used to thinking, "Oh, I always get my milk at So-and-So's, it's cheaper there," You really need to pay attention because at any given moment, milk might be cheaper at Such-and-Such this week, so unless you literally stop in front of every item and check the price for it at every store in town on your cell phone, you'll never know if it's actually cheaper here or there.
So in addition to being gas-expensive, the bargain hunting strategy is also a lot more time consuming and requires you to constantly keep track of price fluctuations in your head. People who are shopping for their family do not want to spend the whole day going to multiple stores and then spend ten minutes double-checking the prices for each individual item.
It makes a lot more sense to do ALL your shopping at Costco and not waste time worrying about something being a dollar cheaper someplace else because you're earning cash back and fuel points.
Finally, and this is key, Costco is VERY smart about their free samples and their food court prices, so it's really easy for a harried mom with multiple children in tow to feed the whole family there while she's shopping for the rest of the week.