Because the existence of these people means that we concentrate economic resources toward building yachts that might otherwise be concentrated toward growing food or building homes.
Rational economic actors who produce goods and/or services will tend to supply what there is the most economic demand for (what brings them the most profit). If half the population has almost none of the money, then their needs have little economic demand behind them. So then what is the economic incentive to supply them with the things they need?
In other words, producing a hundred-million-dollar yacht is a hundred million dollars (less profit margin) that could have been invested toward a more collective good, thus increasing the supply of those goods, thus reducing their scarcity, thus reducing their price.
Rational economic actors who produce goods and/or services will tend to supply what there is the most economic demand for (what brings them the most profit). If half the population has almost none of the money, then their needs have little economic demand behind them. So then what is the economic incentive to supply them with the things they need?
In other words, producing a hundred-million-dollar yacht is a hundred million dollars (less profit margin) that could have been invested toward a more collective good, thus increasing the supply of those goods, thus reducing their scarcity, thus reducing their price.