Most DG are out in more ruralish areas where there's simply less choice, and/or the cost of exercising that choice is more pronounced. Drive an extra 15-20 min to save 30c on milk? Probably not worth it to many folks (cost of gas is more, might make you late for work, etc).
Had a lot of discussions with the former manager of our local DG, and... while not 'eye-opening', it was interesting to hear the pressure she was under. $x/week budget to schedule all the staff, targets to be hit (dollar targets, merchandising display targets, etc). It's "retail", but the numbers she had to play with were pretty low given the volume, imo. FWICT, she wasn't on more than perhaps the equivalent of around $13/hr, although I do think there were small performance bonuses thrown in. But, I think she was 'salaried', so lots of 50-60 hour weeks for probably not much more than ~$35k?
This is a somewhat ruralish area, and this was also... 4 years ago? She's moved on since then, and we lost touch so I don't know if things have changed much with the newer managers or not.
FWIW, she did keep that store ticking over. She was a bit bristly at times with other staff, but ... there was a pretty noticeable decline in staff behaviour, sheving, cleanliness and overall experience at the store within a couple weeks of her leaving, and it's never quite recovered. I'm guessing had they paid her an extra $100/week she'd have stayed and continued going above and beyond, but... .hey... profits...