It's a good point. The logic lies in the fact that companies continue to innovate and increase productivity, in addition to future generations making even more money, and putting that into the market. It IS a zero sum game, and once both of these two things cease happening, there will be a financial crisis. Look at GME, they didn't innovate, but people who weren't invested in the market put their money into it, and made it popular. That's a pretty clear bubble and is essentially gambling. But if you look at the overall companies in the US, say the S&P500, investing in that is essentially saying "I believe new innovation will come where other investors want to invest". Of course, then other investors think you are doing the same thing, and it becomes a bit of a random cycle with asymmetrical information.

Yes, it's all made up. No, don't question it. Lots of things are made up and work well for us. Money in general is made up. Culture is made up. But the fact that many people have faith in it, brings people comfort. Keeping your money in the market instead of selling it is good for the market, and gives you financial security. Hopefully there doesn't come a day when everyone cashes out.