Why do you assume that someone would get rewarded for slacking off? Remember it's "from each according to his ability", not "communism means free stuff for everyone".

I get paid a stupid amount of money to sit around in my underwear and type on a computer all day.

Meanwhile, I know people who are awake at 4AM so they can work a handful of physical labor jobs. They don't get home until 8-9PM and weekends off rarely exist.

One of my friends also does physical labor, but is unionized. He has one job and weekends off. He also has health insurance. He is no more skilled or unskilled than my other friends. The only difference is that he's in a union.

Employers base compensation off of market forces that often have little to do individuals and everything to do with the fact that labor's been commoditized.