Believe it or not, some people actually are willing to work harder for more money
People have literally fought and died for the right to unions, for unions to be taken seriously. A bunch privileged silicon valley tech workers (not me, I'm a privileged seattle based tech worker, totally different ;) ) write off unions as inefficient and useless.
Sometimes harder workers are harder to find, so they get paid more. Sometimes they're not. If working harder simply led to more money, salary negotiation would not be a teachable skill.