Their income is not free off the backs of others.
They must work to earn the money to invest in the company and the return on a single share of stock is negligible, so they must work more, to have the resources to invest in the company to earn more money.
My companies share price is usually around 25$, their annual dividend rate is $.63, how can you call that a "massive" return?
I must put up $25 to get the return of $.63.
Shareholders provide a benefit to the company, loaning money for the prospect of a return, not a guaranteed return but a risk appropriate return.
A worker only gives his labor, largely risk free and is paid in near immediate terms.
A worker does not give his additional resources to finance the buying of materials, machinery, land, structures, advertising or anything else.
It is, though, an example of mutual aid.
Not very far away from communal ownership, which is what a shareholder is, a communal owner.