Wealth is NOT fixed

I have been surprised over the years by different people proclaiming that the amount of wealth in this world is fixed. For example, the U.S. needs poor countries to be rich. If the poor countries increase their wealth, then the U.S. would have less wealth. I have heard this kind of illogic over a dozen times. I think the situation is such because of two reasons. One, the economics teachers are not good enough and second, most people aren't interested in economics and simply have iron-clad, mercantilist/marxist economic, beliefs. Another example of said economic fallacy. The total wealth in this world has been increasing for decades now. The U.S. would, in fact, be wealthier if more countries in the world were richer. Because then these richer countries could buy more U.S. products and vice versa. It's a win-win situation not a zero-sum game. We see such economic illogic in a different context as well, where one person/group's financial gains must come from the loss of another person/group.