Because the main way money moves in and out is by people buying in and cashing out. If no company injected more (by dividends and stock buy backs) and no company took money out (by IPOs, funding rounds or stock options), it would just be a box people would put money in and get money out.
ln that case it would be a zero sum game, because every dollar taken out by someone was put in by someone else, but because there is a flimsy connection to companies creating/extracting value in the real world it is not.