Sad truth is that most people do buy grocery store pet food. Either they don't care or can't afford anything else.
Most of my friends who buy grocery store food just don't know any better. When I try to educate them, some of them tell me their dogs are doing fine on their current food (don't care). Others have switched to a better food - yay! Also, some dogs don't do well on one of the pricier foods - they're too rich for them.
My boxer puppy, Brodie, is an example. None of the better foods agreed with him (TOTW, Earthborn, and Natural Balance), and I think I've narrowed it down to chicken and potatoes that he's sensitive to. Many of the better kibbles have potato in them, or multiple protein sources. Instinct has a couple of potato free/single protein source kibbles, but Lord have mercy, they're terribly expensive ($70-80 for a 25 pound bag - so yea, too expensive for me!!), and they also have clay in them (??). Brodie's now slowly transitioning to Holistic Select Lamb and Rice, and so far, so good. It doesn't rate as highly as some of the others, but at least he seems to be tolerating it, and it's better than the cr@** Rx diet the vet has him on. Having a sensitive tummy dog can be quite challenging! Luckily, my minipoo has a cast iron stomach and can eat Brodie's food rejects - lol!