If you spend 50% of your income every year and stick the rest into the stock market and get 6% or so after inflation, you can sustain your spending without working after about twelve years.
If you spend 90% of your income every year and stick the rest into the stock market and get 6% or so after inflation, you can sustain your spending without working after about forty years.
It's all to say that being a shareholder matters in a hugely different way depending on your income. The disparity is worse when you look at how you can sustain higher volatility once you're already wealthy.
You're asking why can't people just be shareholders, but I think you're asking why it isn't enough that everyone can be shareholders. The reason is that being a shareholder only matters for those already making a ton.