I think there's a second element to why a gallon of milk isn't $20: Walmart relies on not upsetting too many people, who collectively wield enough local politic

I think there's a second element to why a gallon of milk isn't $20: Walmart relies on not upsetting too many people, who collectively wield enough local political power to harm Walmart locally. It would only take a local wage initiative, for instance, to likely bother the closest Walmart. So there's an inherent incentive not to let prices rise so much that people are angry enough to get the government involved because they can no longer buy milk (or food).