Let's talk about the argument that life has diversity without a narrative reason, so it's obvious that all fiction should have diversity of race, sex, and gender.

The problem here is the assumption that good fiction reflects reality–all fiction needs in order to be effective is to be more stimulating and fun than everyday life. Ever recorded a conversation at a dining room table? It would be full of repetitions, mumbling, and pointless agreement. Good dialog is confrontational, succinct, and indirect. Ever tried to record your Tuesday and make it into a novel?

The writer's job is to distill something commonplace into an experience for the reader. And to do this, the writer needs to have a reason for every word they put on the page.

Depending on the audience, the genre, and the story, a book may have every important character be males of one race, and it will work (Old Man and the Sea by Ernest Hemmingway comes to mind). Or a book can have diverse and quirky characters, and it will work (An Absolutely Remarkable Thing by Hank Green).

We are built to notice discrepancies. When you see a sheet of green paper with a yellow dot, you first notice the dot. When the reader reads a book, the reader is the green sheet of paper, and they overlay the book on top of that sheet and single out the people that aren't green and ask why they are in the story. I think they deserve a better answer than "I see this every day, so of course I put it there".

This works both ways though. If the reader lives in New York, and they read a book where everybody is white, they will think that the author intended to deny racial minorities their existence–why else ignore all this stuff that's in the reader's life? Because their sheet of paper is multicolored and a monochrome book is obviously wrong when placed over it.