Why don't nearly half of Americans have any investments?

Because their income does not fund basic needs which leaves no real power to invest.

That is also why very large numbers of people do not participate in many areas of the economy.

In the late 1980's I took a (mandatory) economics class in high school. Is that still done? I'm not rich but I have a little saved some that will be very helpful in the not to distant future (learning and doing are two different things, I could have done much better.

Disposable income is no doubt a factor. Those "just scraping by" do what they can to pay the bills but I think for many a little better off it is just the amount of stuff we buy. Lots of reasons of course but part of my take on things.

Because we're struggling to eat

"Because they do not have enough money to invest." Sorry.