It's because courses that teach how to make a fortune publishing ebooks (Fiction Profits Academy, etc.) suggest that wording. I don't know if the students are specifically told to advertise for ghostwriters and editors on Upwork because there will be people willing to work for very little (or if they are told to say they're just starting out and don't have much money--which they probably don't after paying $2000 tuition), but there's a whole lot of lowball ads out there and they're all worded the same.
This also explains why these clients don't realize they're often paying for absolute garbage writing (to paraphrase Gore Vidal's famous quote about Jacqueline Susann--whose work I enjoyed--"they don't write; they type." Those barely readable manuscripts are then turned over to an editor to clean up, and the clients believe they can get this done for a pittance, not realizing that the ghostwritten manuscripts are usually of poor quality (especially when the freelancer is told they must produce 15k words a week) and that the only people who will agree to "edit" these don't know an adjective from an adverb. No skilled editor would even consider spending two or three days editing--really more like rewriting--a badly written 20k manuscript to earn $50. But the people placing these ads don't know the first thing about publishing, they just want to make money.
I always thought clients were just copying the wording of other clients' ads, but now I know that's not the case. That one course I mentioned charges $2000 tuition. This is like those medical transcription schools that charge high tuition to train students for a field that is dying and no longer pays a living wage because doctors and hospitals are using voice recognition, except at least that course doesn't involve trying to get a product created by other people for a pittance...
