The Left's argument is that in things like wages the outcomes should be equal regardless of the effort or skill involved in getting them. The Left is all about equal outcomes (a participation trophy for everyone). The non-Left is about equal opportunity to get an outcome. If the results vary based on hard work, skill, determination, etc., then that's fair because those that put in more useful effort get rewarded commensurately more than those that were stupid, lazy, unmotivated, or whatever.
With the Left, they want to drag society down to the lowest common denominator rather than build society up on the shoulders of those that can and do put in the most effort.
Everybody but the Left gets that when you grow up and become an adult, you are the one responsible for getting stuff done. The Left wants to remain children and have somebody else--like the government--take care of them so they can spend their life in idle leisure and doing useless crap...