Solitary by Necessity: By definition, writing fiction must be a solitary affair. When it comes to non-fiction, a collaboration between writers is possible, often highly desirable.
In this case, two heads are better than one when it comes to the necessary amount of research required. Whereas the very idea of attempting to co-write a work of fiction with another writer leaves much to be desired. It only ever works in the arenas of film and television, where a team of scriptwriters brainstorm and throw ideas for specific situations into the mix.
As you begin writing, it gradually advances from the embryo stage. Like any prospective parent, you jealously guard and nurture it as it grows. So the very idea of sharing that idea with another writer, expecting them to feel that same way about it, let alone using your ‘voice’ because you are the originator, simply does not compute. The idea is yours and yours alone. Therefore you must always be the one to see it through to completion.
As far as books go there is only one area in fiction writing where any form of collaboration appears to work successfully, and that is in the field of short story anthologies. But even then, each tale is written by a solitary writer, so it’s still not true collaboration.
Having said all this, I do know of one recent example where two fiction writers worked on one project together. Whether or not their brave, some might say foolish, decision to collaborate, will be justified is now down to the reading public’s acceptance of the book in question. After all, as writers, we are all aware of just how fickle, nay contrary, the reading public can be at times.