I am a happy customer of both a bank and a grocery store. If one day the grocery store decides to also open a bank, I won't suddenly move my accounts there just because I go to the grocery store every week. That's why the grocery stores trying to offer credit cards usually have some loyalty programs (e.g. points/discounts for in-store purchases). And BTW, they typically just partner with existing banks and not try to butt into a heavily regulated market with no previous experience.
And if I never needed to have a bank account before, just being able to open it while paying for my food won't suddenly create the need for it.