I'm going to answer this in the simplest way possible:
The stock market is not a zero sum game because in the past few centuries (from industrial revolution to now), there has been a consistent, net positive inflow of capital to the financial markets.
Before the industrial revolution the economy was a zero sum game. If world GDP was indexed at 100 in the year 0 AD, it only went to 200 after the year 1800. Everytime there was economic growth, wars, famine, etc made sure growth always mean reverted after a couple of years so the average stayed at 0% for nearly a millenia. THAT was the zero sum game.
The stock market is not a zero sum game because in the past few centuries (from industrial revolution to now), there has been a consistent, net positive inflow of capital to the financial markets.
Before the industrial revolution the economy was a zero sum game. If world GDP was indexed at 100 in the year 0 AD, it only went to 200 after the year 1800. Everytime there was economic growth, wars, famine, etc made sure growth always mean reverted after a couple of years so the average stayed at 0% for nearly a millenia. THAT was the zero sum game.