In almost no category that I've sampled has Walmart ever been the absolute rock-bottom cheapest compared to several places (Big Lots, Dollar General, local grocery chains, Amazon... even Target, believe it or not). People, however, can't go to Amazon to get infant formula, a car battery, ammunition, and a jar of pickles at 1AM and have it in hand immediately.

Walmart used to be about "made in the USA", then it was "always low prices", now it's about "live better". It's Walmart's ubiquity, convenience, and momentum that keeps them dominant (and their ability to loss-leader competitors out of business).