They tend to be somewhat upscale in terms of ingredients, employee training, overall experience, etc.

That's not to say that Burger King or McDonald's couldn't do better. I don't really have enough experience with them as a customer or anything else to have a real opinion. But I suspect that, if Burger King were to try to improve their experience in a way that forced them to raise prices by 25% they'd lose a lot of business from people basically just looking for the cheapest burger.

See also the experience of JC Penney and Ron Johnson. A lot of people are pretty much in the position where they have to shop on price.