Individuals negotiate based on individual skill and experience, not unions or any other collective structure.
As far as I know, unions in for example switzerland focus more on setting baselines for skill/experience, but they are also much less prominent than in places like the U.S.
The same reason why minimum wage laws always hit those who are at the bottom, they are no longer able to get a foot in the labor market for various jobs as they can not enter a minimum
The result is the same in the sense that it sets up artificial barriers to entry by pricing out entry into the fields for those who are at the bottom, it is a way to limit competition.
