Employees who negotiate higher salaries are subsidized by their meek coworkers.

They can afford to pay those who negotiate closer to their true value (often still significantly less) is because they pay the vast majority of people less.

The only way transparent salaries help the majority instead of being used as a tool for businesses to just say, "We only pay 100k/yr, that's what everyone makes" is if there is some body enforcing fair pay.

A manager could then point to the salary of someone in the same position and say

"Jane gets paid $X in the same position, it would be unfair to give you a raise and not her"

I don't work somewhere with transparent salaries and I already get that excuse — I've more than once heard "you're kind of already at the top of our range for X position" and "if we pay you anymore you'll be making more than me!"