Because so far, no-one (within the US legal system) has come up with a formulation for "monopoly" and "market" that does not have (a) unpleasant side effects fo

Because so far, no-one (within the US legal system) has come up with a formulation for "monopoly" and "market" that does not have (a) unpleasant side effects for businesses that would normally not be considered monopolists (b) a system that is easily gamed by businesses that are trying to monopolise a market. Not saying it can't be done, just that it seems to be fairly hard.