This is very much not true for worker salaries. For one thing, workers can not easily hop from area of expertise to area of expertise like capital can — and workers oftentimes lack the ability to take the high risk that might comes with quitting a job in search of another or attempting to switch career paths.
The natural state is for owners to drive down worker compensation until it's essentially slave labor. In a "competitive" world for owners, competing means driving down costs which means an across-the-board destruction of worker benefits.
We have some protections in this country (thank you, unions!) and full-stop rules against slavery that prevent us from actually seeing this, but that doesn't mean it wouldn't happen.
And trust me: People in financial power would absolutely grind workers down to a penny an hour and still claim it's "fair pay" if they could. They'd say "well, if it wasn't fair pay people wouldn't do it" while making sure the entirety of society was arranged to prevent people from leaving their jobs.