The reason might be, most people do not like to be in the literal shit of others? It comes with actual health hazards btw.
"and there wasn't enough supply to meet demand, then you invest more in technology that reduces the shortfall. As others suggested, automation"
But we ain't there yet at all. What do we do, till then?
The sewage needs to run 24 h and not only if someone feels like taking a look eventually.
And as for ordinary garbage: mostly it is not a desk job, but physical labour to touch and move hundreds of different dirty garbage bins every day.
Dealing with that shit, should always come with good compensation. (whether money or social credits or whatever currency is in use)