Wealthy people take more money out of the economy than they spend. Poorer people have to spend most or all of the money they get their hands on so they are a much more efficient force in the economy. Every dollar a rich person makes in interest or profits from an investment has to come out of the pocket of someone else who would use that same money in a more direct way to pay for goods or services. IOW the more money gets concentrated in fewer hands the less efficient the economy gets which means more people have to get less for doing more to make up for the inefficiencies (the rich are not going to be the ones picking up the slack BTW). Also we are a social animal and studies show the more fair and equal a system is the happier all participants are, even the ones who started out with more to begin with.
-Karl