This someone tells me their idea would make a great book.
Yes, the plot sounds entertaining and it probably would make a good movie, too, if they could get a filmmaker interested in it. I've always thought several of my own books would make good movies.
Next, this someone asks if I will write their book for them.
"But it's a terrific idea!" says this someone (who is afraid someone else will steal their idea and make a million dollars with it).
Ideas are all over the place--like pollen in the springtime. For whatever idea an author has cooked up, another author has used the very same idea.
Ideas cannot be copyrighted. (This is a fact.)
Writing a book takes a lot of time. Each word must be typed into a document. The document will then need to be edited. There are many factors involved in a good edit besides punctuation and spelling errors. The book must be structurally sound, dialogue should flow in a natural manner, there should be variety in the author's choice of words, and length of sentences. Verbs should be active and not passive.
There are people who will write a book for someone else, but they need to be paid to do it, because it is work to sit down at a computer and type for hours and hours. It is not an easy job. I give this someone the name of a friend who has a business writing books for other people.
After editing, the book needs an eye-catching cover. Finally, it needs to be marketed, which is the most difficult job of all.
Odds are the author will not make a million dollars.
My someone goes away, rather disappointed.
I go home, turn on my computer, and happily type out another chapter using my own idea.