Even if this were true, so what? If you have to reach the 60th percentile, or the 80th, before Americans have stock amounts, why is that a big deal? It's clear that many Americans who are not the ultra-rich/utterly-financially-secure types own stock, and own stock in amounts that are relevant to their own financial health.
I don't think it's helpful to anyone's understanding of any situation to try to pretend that tens or hundreds of millions of Americans -- who are on the whole somewhat wealthier than average -- are mega-rich snobs who drink ten-thousand-dollar-a-bottle wine.
You need to distinguish between two ways people are affected:
1) Indirectly via mutual funds, pension funds, and so on.
2) Directly through buying individual stocks on E*Trade or the like.
The second amounts to an expensive hobby (in risk adjusted terms). Whether some people who aren't ultra-rich choose to indulge in such an expensive hobby is neither here nor there, the underlying fact remains that there's no good reason to care about or optimize for their benefit. Yet for some reason so many articles on this subject instead of talking about the funds that are broadly and rationally held, analyze the situation for the small retail investor.
I don't think it's helpful to anyone's understanding of any situation to try to pretend that tens or hundreds of millions of Americans -- who are on the whole somewhat wealthier than average -- are mega-rich snobs who drink ten-thousand-dollar-a-bottle wine.