> Why are these "investors" not the common American "worker" whose wages are low?
Because workers don't have money to buy stock, so they don't get the gains from stock appreciation. And because workers are taxed vastly more heavily than investors at the same income level, people who don't stay out living off investments rather than wage labor are, even if m with the same pre-tax income, greatly disadvantaged at accumulating the resources to become others of capital sufficient for their own support (the lucky worker manages to accumulate enough for a modest but decent retirement by the time they are incapable of further wage labor, but many do not.)
It's a lot easier for a modestly wealthy capitalist to become a very wealthy capitalist than for a worker to become even a self-sufficient capitalist.
> Why has the market not created such an instrument to supplement the "worker" who has low wages but could ultimately profit from the ever-increasingly efficient economy?
Because that would involve the people who have disproportionate political and economic power handing over the tools of accumulation of that power to the people they spent considerable effort denying it to in order to accumulate it themselves.
> Why doesn't the stock market act as a great redistributor of capitalistic wealth?
Because that (at least, in the direction of redistribution you propose) is literally the exact opposite of its purpose.