For the theory of monopoly to make sense it has to apply to both macro and mirco, sure from the individual consumer's viewpoint there appears to be monopolies everywhere (Walmart in a small town) but at scale we don't see that, we see vicious competition between enormous firms (and small firms) all over the world, Walmart may not have powerful competitors in a small town, but go to any plaza in the US what do you see next to the Walmart? Nothing but competitors; a Target right there, a best buy over there, a Home depot down the road, a Costco across the street, there's centralization of capital absolutely, but where's the monopoly in this picture?

Now over the history of capitalism there have been phenomenon that take on the appearance of monopoly (company towns, energy cartels, massive nation sized corporations) and sure you can assert there are localized monopolies, but as an aggregate across national economies small firms still exist in enormous numbers, and at the scale where the majority of capital is made, i.e. across national borders there's nothing but competition

Now there's widespread confusion on this matter because neoclassical economics implies that any firm at scale is not competing "properly" and as such is a monopoly, liberals and modern Marxists alike fall for it, but it's a fantasy construction by a tradition that was designed specifically to negate Marxist economics