Building a car is a repeatable process. It happens on an assembly line and each step is the same. There are some small, intentional variations in the process to accommodate customization like paint color and optional components. But for the most part, the assembly line implements repeatable steps which produce the same output every time. There are entire industries devoted to identifying and reducing variations in manufacturing processes so that every produced part is the same. Writing a book is nothing like that.
Writing is generally an act of discovery. As you write and flesh out the details of the story, you often have to go back and rework earlier scenes to incorporate and set up those details so that the whole work becomes cohesive. Writer's tend to group themselves into "pantsers" and "outliners" but it's much more of a continuum than two distinct groups. Even the most dedicated outliners seldom plot out every detail of a work before they start writing. The more detail you put into an outline, the more closely it resembles the finished product. 396
At some point, creating an insanely detailed outline becomes equivalent to actually writing the story. You can put more work into creating the outline so that there are fewer changes to make to the first draft, but that doesn't really reduce the amount of editing you're doing. It just shifts the work from one stage to another.
In addition to editing for plot, there's editing for flow and smoothness of the text. I know of no author who can sit down and write pages of perfect copy. The serene, effortless flow that you get in a great novel is the product of rewriting and editing:
Interviewer: How much rewriting do you do?
<PERSON>: It depends. I rewrote the ending of Farewell to Arms, the last page of it, 39 times before I was satisfied.
Interviewer: Was there some technical problem there? What was it that had stumped you?
<PERSON>: Getting the words right.
— <PERSON>, [The Paris Review Interview][1], 1956
Writing is a labor of love for most authors, but that doesn't make it any less a labor. This thought isn't original to me, but it's one I've found highly relevant:
Do you want to write, or do you want to have written?
Writing is hard work. There are no short cuts. You can improve on some things by practice and repetition, but you will never eliminate the need to edit and rewrite. It's part of the process. 487