I'm not positive, but for one thing, a developer can more easily work remotely than most of the jobs you mention. That makes them more likely to be able to find

I'm not positive, but for one thing, a developer can more easily work remotely than most of the jobs you mention. That makes them more likely to be able to find another job hiring from just about anywhere to compete on pay. A janitor must uproot her life in order to move to the site of a new, better-paying job, and that often comes with additional costs (not only for moving, but also increased cost-of-living).