So do big box retailers and grocery stores not have any competition then? I don't want to be snarky but it's not exactly a mono or even duopoly space in most towns for grocery stores and big box retailers are more often single choices in medium and smaller towns but above that you've often got a choice of big box retailers to go to too.
Since WalMart and dollar stores compete in the same space to some extent they have competition. And most things you buy at target can be bought from smaller specialty stores that while not direct competition between them all you have same same variety of things available. (and different prices and quality levels so you can make your own trade offs)