If you’re rich, others must be poor

There is a limited supply of (valuable) money, commodities, etc. in this world, and most of it is concentrated among wealthy nations, corporations, and billionaire individuals who often do not work as long or hard as people in poorer regions or nations. If “trickle-down economics” works (as Republicans in the US often claim), why are there poor people and nations in the world? Wealthy people can’t be counted on to reliably share their wealth or help others.