Plumbing is hard because it is not forgiving. It's as binary as IT except you can learn the outcome with some delay, once you learnt about a damage caused by a leak. Either you do a pressure tests right or repair can be expensive. And bugfixing is always tricky.
Water also goes down whether you like it or not. Think about all possible leaks inside the shower cabin. Or what is even more impressive that under a pressure the water goes everywhere possible.
Plumbing is similar to electrical engineering, except it usually doesn't kill immidiately (though working with gas is tricky anyway) but requires similar strict mental model to do right.
And when you see a plumber it seems like this person is just a physical worker. So work status misconception must be leveled with money...