Why? As long as there are 3 or more, why care about anything but price and selection? If you can find what you want, then it's just about price, no? At least, it is for me.
Because I rather like having vibrant communities sprinkled with local businesses - places where people go and interact - and a local economy not predicated solely on the whims of the Fortune 500. The end-game of what you are advocating is that everyone stays home and buys everything online or, if they opt for brick-and-mortar, their only option are big-box stores: nondescript cookie-cutter islands of mega-commerce in a sea of blacktop parking lots. I don't want that to be the dominant model, even if it means I sometimes pay a smidgen more. That smidgen more "buys" me a community I want to live in, and neighbors that can afford to live there. There's a place for big-box stores and online commerce giants - I have made purchases at Target, Amazon, and Home Depot in the last month - but I worry about me and everyone else being screwed by hegemony.