There's nothing wrong with janitors, but the work they do can be done by any dumb monkey with almost no training. That's why the pay is so low for the job: anyone can do it, as long as they can lift trash cans and push a vacuum cleaner.
Plumbers are entirely different. They have to get their hands dirty working on some awful systems, but they actually have to know what they're doing, get specialized training, etc. Soldering a proper joint with copper pipes isn't that easy, and if you screw it up, it'll leak later and cause a lot of property damage. Knowing which pipes and fittings to use where is specialized knowledge. It's not something you can just grab someone off the street and train them to do in 30 minutes. Of course, plumbers also cost a lot too, and the ones who are self-employed (rather than their assistants) generally do pretty well financially.
