Fixing the plumbing usually takes much less time than writing a book, so recompensing the plumber for the time spent doing the plumbing in a one-and-done lump sum is much more tenable than doing the same for an author, who might have taken weeks, months or even years writing a book.
Plus fixing the plumbing usually only needs to be done infrequently, whereas a book is read in a comparatively short time, so from that point of view a consumer would also be willing (and able) to spend more per plumbing fix than per book.
So consequently you need some sort of arrangements that allow for splitting the necessary payment to the author up across multiple people and/or over time.
Additionally, artists often speculatively create works without knowing for sure whether the public will take any interest in their work, or not. Copyright certainly has its faults, but it does cater for precisely that scenario by ensuring that you can insist on getting paid afterwards if people enjoy and want access to your work, and you don't need to acquire all the necessary funding up front. If you can't come up with enough money, you can even "just" invest your spare time instead and still get paid back if the work turns out be successful.
Plumbers on the other hand I assume rarely have the desire to speculatively fix up other people's plumbing and then hope to get paid afterwards if they did a good job.
Rewarding artists after they've already produced the artistic work if they're successful also makes sense in that the quality of artistic output can vary, and so there's a bigger risk of disappointment if you need to pay far in advance, before the work has possibly even been produced.
And because the quality of an artistic work is also very much a subjective matter, it'd also be much more difficult getting your money back in that case, whereas plumbing can mostly be judged according to much more objective standards, so getting your money back – through the legal system if required - is again a more tenable affair.
Of course the existence of Kickstarter and the like or even just plain old pre-orders show that to some extent people are willing to take that risk of paying in advance, but whether that would be enough if it was the only reasonable source of funding for artistic works? It'd also mean that if you can't convince people to pay you in advance (and good luck with that if you're some unknown newcomer), then good luck getting any more money afterwards, even if the book/… then turns out to be wildly popular afterwards.