That is a good point, but I think it comes down to the barrier to entry - it's easier to become a janitor than it is to become a programmer so one could probably assume there's always going to be a steady stream of people who are willing to do janitorial work.

> This implies that janitor is an inherently inferior job that deserves lower pay

It does unfortunately. Given some of the awful stuff janitors have to deal I believe they're deserving of higher pay, but unfortunately the job market doesn't agree with me on that.