It is a prejudice. You have to ask them. Ask nurses why they do what they do. Ask house cleaning stuff why they do it. For sure, a lot of people stay in their jobs because they don’t have any opportunity. But if you go deep enough you find out that a lot of them found some sort of fun while doing their cleaning-public-toilet-sort-of-job. (I have done my research so I know what I am talking about.)
The real test for your theory is what people do when they're not getting paid. I have never in my life heard of a large forum/subreddit/WhatsApp group for cleaning toilets. There are no toilet cleaning meetups.