As large as Walmart is, it is still absolute peanuts compared to the scale and (especially) dynamism of global production and consumption as a whole. Global supply chains have to change much faster and in arbitrary ways, compared to the centralized chains of something like Walmart, which in turn is also still subject to the external pressures of competition – even just hypothetical competition based on some hypothetical course of action is a powerful constraint.

Lots of reasons, but here’s one:

Because one of, if not the main purpose of money is to provide a decentralized way of transferring information about economic needs and capabilities. Without that mechanism in place, the only way of determining where goods can be created and where they need to go (a massive problem that it is a daily miracle we don’t generally have to deal with) is by an overbearing authoritarian state.