I bet most billionaires start a company (or inherit from somebody who started a company) rather than buying stock in a company. Venture capitalists or others who provide seed money and get stock don't count as people who buy stock because regular people can't do that.True, but most of the billionaires did exactly that, hold a few stock that perform very well, and stay along for the ride. Index funds are the way to lessen risk, but your are never going to make millions off an index fund, which is the goal of many.
Also, owning total market or S&P 500 index funds is the safest and easiest way to become a millionaire. Put your money in early, keep putting it in, don't panic sell when the stock market goes down. It might not be exciting but it works.