Citation needed. I believe "almost every" is pure hyperbole.
A quick search found a poll indicating 52% of Americans own stock.
http://www.gallup.com/poll/190883/half-americans-own-stocks-...
If you have better information, I'd like to see it, please.
Keep in mind that "owning stock" isn't well-defined though, that could mean (a) own individual shares (narrowest), (b) owns shares or ETFs (broader), (c) owns mutual funds.
But that still doesn't address my point. It's a lot bigger when both private sector, and public, pensions are included.
This issue is near and dear to my heart because I grew up in Illinois (now living in California) and am watching the budget battle unfold there. They have a huge pension funding gap, and the returns of the stock market are going to have a very direct, real effect on how much tax the state has to collect to fund the pensions.