Comment by mitthrowaway2

Not the OP, but one reason is because concentrated wealth allows you to adjust the rules of the market or suppress competition. Another reason is that the market only rewards you for delivering value to people who can pay for it, so wealth concentration skews the production of goods towards a small quantity of luxury items, which lack the economies of scale for efficient production. This may still be a Pareto efficient market but not one that maximizes national wealth or welfare.