Yes they would.. except that most places where there is a walmart, there is no costco. They do not have other higher playing places that can accommodate the workforce. I currently live in Seattle and when I drive down to Portland, there is maybe 3 costcos along I-5. There are a tons of more walmarts, targets and other big box stores. Walmart is the largest non-federal employer in US.

Do you really think people work at Walmart because they love the job? Check out /r/target for some insane stories. If forced to choose between insane working conditions or take their chances, some are willing to take their chances.