> Shareholders receive power proportionate to their buying power. Citizens get a single vote.
Historically, there did exist experiments that not each person has the same voting power (for example the Prussian "Dreiklassenwahlrecht" [three-class franchise]):
> https://de.wikipedia.org/wiki/Dreiklassenwahlrecht
Depending on the amount of taxes you paid, you were assigned to one of three classes. The sizes of each of these classes were chosen so that each class paid 1/3 of the whole tax volume. The votes in each class elected representants for this class.
The idea is obvious: those who pay a lot more taxes should have more influence.
Thus: each citizen has the same voting power is just the "currently fashionable" implementation of democracy.