Ever since we were born, we’ve been exposed to multiple zero-sum games. It is very common to be involved in competitions. In them, by its own definition, when somebody wins, somebody else has to lose. That’s why most sports and games are called zero-sum games, because when you add up the results of all players, you get to zero. Investing is different.
I'll give a few thoughts as to arguments that could help.
1. As long as the market is increasing, investing is not a zero-sum game. In this equation, there's a net gain over time so the majority of people involved are winning assuming you stay in the market. The next question is why is the market always increasing? At the root of it, businesses (and in turn, the economy) are making more money over time. This is because the output of an economy is the number of hours worked x the output per hour. As technology gets better and better, productivity (i.e. output/hr) increases. So, over time, the number of people on the planet have increased and the productivity has rapidly increased. With AI, our productivity will continue to increase.
2. Leverage provides the market with more capital for growth AND allows the compounding effect to take place more rapidly over time. The total world debt-gdp ratio is something like 3.2. While this could clearly end badly, it hasn't yet. So, debt has been 'artificially' fueling the stock market returns over time.
I think the key question here: is it a given assumption that the stock market will always go up? Avoiding catastrophic disasters, the answer is likely yes for the world stock market. As for the US stock market? I think it depends on how it competes over the next century with rest of the world. Eventually, it will decline but nobody knows when that is.