One of the benefits of the government-subsidized interstate highway system is that products can be moved cheaply by truck from ports of entry and factories to wholesalers and from there to retailers, no matter where in the country those wholesalers and retailers are located. Another benefit is that it's economically feasible for a family to live in the suburbs, work in the city, and shop in a more distant suburb.

I wouldn't give the government "credit for Walmart", but the government has created an infrastructure that Walmart used to its benefit.