Why do the stonk go up?

It's seemed like the more I learn about stocks and investing, the more I don't understand. Why is it that the common advice is to invest in the S&P 500 and in total market indexes? What's the overall idea that the prices for said products will (most likely) indeed continue growing?

If the price of the S&P 500 has continued growing, does that mean that these companies are overall worth more, and thus the overall stock market is indeed worth more? Or are more people putting more money into the market than in the past, so these prices are inflated? Are these companies really that much more economically productive (accounting for inflation) than the companies 50-80 years ago?

More research pending!