This assumes having money to invest in the first place, over and above the necessities of living. Investing implies risk, in other words, being able to lose that money. More and more Americans have next to no savings and are living paycheck to paycheck. They can't afford to invest and gamble on the stock market.

I repeat here Sam Vimes' "Boots" theory of economic inequality (from Terry Pratchett's Discworld novels): "A man who is wealthy enough to be able to spend 50 dollars on a really well-made pair of boots will have a pair of boots that lasts ten years. A man who can only afford a 10 dollar pair of boots, a horrible wreck that leaks with soles that have to be filled in with cardboard and wear out in a year, will have spent twice as much on boots over the course of 10 years as the wealthy man, and will still have wet feet."