When I did the equivalent "lets learn about stocks" at school, we opened the newspaper, found a stock, "invested" fake money in a ledger the teacher kept, and then cashed out at the end of three weeks after doing some rough calculations.
The equivalent for a real world person would have been to
1) know what a brokerage was
2) find one
3) drive there and fill out some paperwork to make the account
4) deposit a paper check or cash at the location
5) get a phone # with a specific broker at the brokerage
6) subscribe to a local paper with the stock section
7) watch one of several thousand stocks, and write down by hand the movement of the stocks to see if you could set a strategy
8) maybe buy subscriptions for some investment magazines, a few hundred dollars per year
9) decide the time is right, call your broker, leave a message with his secretary to get back to you
10) wait for that to happen, then place your order. your broker would then charge you a percentage (up to 3% of your trade!) as their fee
11) Angry men in a pit in NYC would yell at each other for hours to make the trade. The trade would be communicated on scraps of paper with hastily written numbers, then shouted over a phone on the trading floor
12) wait for your broker to send you physical mail with your certificates or a record of ownership showing that the trade closed several days after you placed it with him
13) go through a similar process to sell your stock certificates, but then have to self track capital gains for your taxes
Can you name a single reason why you think a poor person living paycheck-to-meal would even know about or wish to participate in this?
Things got better with the internet, which didn't exist for most people until I was an adult (I know because I helped start an ISP), it still cost $30 trade electronically and settlement still didn't happen for days. So somebody making hundreds of dollars a month would have to lose $30, just to make a bet of whatever they were able to scrimp together over months, in the hope it would turn into more than $30 so they could sell it and make money above the trading fee.
Again, why would a poor person, who may not even have a bank account, be a participant in this?
So, to help with your incredulity, why would somebody, who is from a poor family, with not a single person around them trading stocks, most without full-time employment, take a brief class when they were 10, retain those precious handful of classroom hours, until they get lucky enough to have disposable income decades later and suddenly decide "I'm going to trade ETFs"
Most people don't work in companies with a 401k, or even with any benefits at all. Why would they be familiar with managing one? Next time you eat out, trade some tips with your waiter on how they diversify their retirement investments. Ask them which ETFs they like, and if any have especially low fees, or do they pursue a market segment strategy. What's their opinion on I-bonds? Maybe they have a prediction if Cathie Wood is a cook or not?