Advocates of capitalism attempt to hide this by claiming wealth creation is not a zero-sum game, as if investment somehow magically produced wealth.
Another red herring is the idea workers enter the transaction voluntarily, but most people--in a purely capitalist arrangement--obviously would have no choice but to work to survive.
Another major reason is the fact that capitalism prioritizes production for the benefit of the wealthiest first, and the neediest last (again contributing to its ultimate unsustainability). Anyone initiating production (starting a business, etc.) will generate a product or service for those with the most wealth available (a market). It would be irrational under captitalism, to, for example, start a business feeding starving people who have no money.