Zero Sum games create nothing. A gain means another's loss. Wealth is captured rather than created. Mercantilism was the system in place as global trade really kicked off. At the same time as nations were starting to be created (16-18th century), there was a belief that trade was a way of getting powerful. One state does well at the expense of another. Nations wanted to maximise their exports (subsidies) and minimise their imports (tariffs). Forbidding colonies to trade. Forming monopolies. It took Adam Smith and others to show that this was silly. When wealth is created, both partners to a trade can benefit.
Win Win > Win Lose