Not by choice. Stocks are pushed onto Americans (as well as Europeans, and most people in the West) via their pension funds, advanced savings accounts, and sometimes even their salaries (via options). If normal savings accounts in their local credit union would offer adequate interest rates (and if paying in stocks was outlawed by their unions) stock ownership would plummet. I would be surprised if it would even exceed 2%.

And how many of those people are actively making decisions about what companies they are investing in instead of blindly putting money into a black box 401k account because they are financially punished for not doing so?