It is different that plumbing/carpenting et al are one time gigs. They just walk away collecting their money once they put in their fixed hours. You will not be able to call your plumber at midnight if you feel like some thing that needs to be fixed.

Your analogy might work for short term contractors but not for a full time employment, where essentially you are putting yourself at your employers service virtually 24/7 with almost no extra pay while doing intangible (aka knowledge) work where the employer might get 5x or 200x return out of your work, but essentially you will just get almost linear pay raises (even including performance bonuses).

You cannot compare a full time knowledge worker with a gig worker that does tangible physical work.

Plumber's job with a single customer usually lasts one or two days, so the plumber's income is not dependent on any single customer. They might be willing to risk it and take a job without a price set beforehand, because the worst case scenario they lose, say, a single day pay.