Why are fiction books generallly written by a single individual?

I think that most fiction authors probably consider their work a form of art. Creative control and self-expression are probably the main reasons. I've considered co-writing a scifi with my little brother as we're very like-minded in that space, but I am not likely to ever consider anyone else - I'm not interested in just producing something, I want to use my creativity to make something unique.

Once you start outsourcing any part of your work, it's likely to become a product rather than art.

What I find strange is this.

Marvel comics from the Stan Lee era were created by small groups that included a writer, an illustrator, a colorist and a letterer. The illustrator usually contributed to the study but Stan Lee had a special talent when it came to choosing specific words. (Look at books Jack Kirby did on his own to see this.)

A team like that could produce several books a month.

Modern Marvel and D.C. comics are produced by a different process. You might see a series of 12 different Superman books that have 12 different creators in a much less economical art style than the Stan Lee era. They go at parallelism in a different way.

Japanese manga creators, on the other hand, seem to be amazingly productive. Allegedly books like Sailor Moon, Gintama and Jojo's Bizarre Adventure are written by a single author.

The rate at which manga creators work isn't fast enough to support an anime series, a problem which is explained brilliantly in this clip from Gintama

https://www.youtube.com/watch?v=a4S9NuI6NKo

Here's something that might apply in some cases: a group of writers might well be capable of producing a better fiction book than a single writer but probably the process would be less efficient: with all the discussions, three writers might take longer to produce a book than one writer would. TV and cinema have big enough budgets to justify the increased cost, but books don't.

This can quickly lead to "design by committee" or "too many chefs in the kitchen", where the result often feels watered down, may suffer from consistency errors, or may lose focus as each writer feels that some part of the story needs to be told differently or that one part needs to be emphasized over another.

I just finished reading one book which I am sure would have never existed if it had been written/produced by multiple people.

Lolita by Vladimir Nabokov. A masterpiece.

It took him many years before he was able to find a publisher (a French one specialized in pornography novels)

Some of his relatives were shocked by the book.

I am sure, it would have watered down many, many times.