Costco has introduced self checkouts. However, to leave the store, you have to show your sales receipt to a checker who matches the count on the receipt to the count of items in the cart. It’s hard to beat that system, unless one hides lifted items in their coat with deep pockets. However, items at Costco are mostly large bulky items; which are hard to hide in a coat.
That system gets beat all the time. Thieves print their own bar codes, or reattach bar codes from other items with lower prices. The employee checking receipts finds the correct count but doesn't know the prices on to check them.
Checking receipts at the door is part of your contract as a member of Costco.
Walmart and other stores may post someone to check, but you are under no compulsion to stand in line bleating like sheep waiting for the guy to check your receipt. At this point, the items in your cart are you property and the store has no right to search your property.
I just do my best Dionne Warwick imitation. “Walk on by.”