For example, the janitor probably has concrete and relatively unchanging definitions of success in his work. His job has tangible effects every day on those he provides his service to. There aren't many janitorial meetings. He probably doesn't have to massage the ego of his boss. To my knowledge, there are no Sanitation Reviews where some coworker will criticize him for not following the One True Sanitation Style or the latest fad in Toilet Driven Development. His profession is not constantly put in the critical spotlight by the media as a front line in the culture wars. There's no such phrase as "janitor bro" which people use to stereotype and demean them.
Focusing just on "I make $X, they make $X/4, how can I complain" downplays a lot of conditions which can make some people in this line of work very bitter about it.