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The problem is that humans are value-maximizing when we leave them to their own devices. If the work that I do directly relates to the value that I can receive (capitalistic) then each person decides how much work they want to do to maximize their personal value calculations. If the work I do does not result change what I get (communism), I maximize value by reducing my labors and receiving the same number of goods.
It very quickly becomes a race to the bottom, where each worker is trying to figure out how little work they can do and still get paid. This does not make for a good economic system and we have yet to find a way around this most basic problem.