Because the stock market doesn't represent the economy as most people experience the economy. First, a lot of companies don't pay out dividends or buy back stock these days, so as time passes, removing their stock price from the price at IPO, their stock price becomes based on perception--not even perception of the reality of the company's value, but perception of the stock's value, which is increasingly just specula…
John Maynard Keynes developed this idea (that came to be known as Keynesian beauty contest[1]) in 1936. This isn't a new property of the market, it has always been the case.
> when companies do pay dividends or buy back stock, it's sometimes done by borrowing money
It's not clear that this is a problem, given that cash is basically free (though they do have to pay back the principal). I'd be interested to see what proportion of dividends and buybacks comes from borrowed cash. I suppose calculating such a thing would be very difficult, but it'd be interesting to see some analysis on this.
> if 90% of people pull out of the stock market, it's quite possible for the stock market to go up
What if 90% significantly cut consumption? Ultimately the companies have to sell their products to somebody.