What is quite apparent is that writing a book requires a team of people. Rarely does one person have a complete skill set to handle the task.
Writing a book entails:
Having the imagination to envisage the novel.
Having the ability to create a plot.
Having the writing ability to create interest in a reader.
Being able to invent characters.
Being perceptive to see flaws in the plot.
Having the knowledge of grammar, punctuation and spelling to be able to correct mistakes.
Possessing the ability to make the language flow and create pace.
Being able to describe the novel in such a way as to create interest without introducing spoilers.
To possess the artistic skills to design a cover.
To build up the social media connections and other media connections to market the book.
Creative people rarely have the objectivity or skills to redraft, edit or see the flaws in their writing. They require a methodical editor to point out necessary improvements and corrections.
A person skilled at writing may not be at all skilled at design or even able to create an enticing and succinct back cover blurb.
Building up social networks, writing press releases, doing book signings and developing contacts, takes time. Most writers would rather be writing and might well be hopeless at communicating in other ways.
A team can hone and present a book to optimise its potential.
Writing a good book and selling it requires a good team. That’s why writers form relationships with publishers and Literary Agents.