Because it doesn't, well, it does, kind of, most (non-US) libraries class all fiction under one 8xx code. Some US libraries use 813 which is specifically 'American Fiction', although the rest of the world doesn't split American and non-American fiction, because it's a largely pointless distinction.

8xx is generally 'Literature' but it is mostly split into different aspects below that, analysis, writing guides, poetry, etc.

The problem with categorising fiction is that it largely comes down to genre - other than the '813' classification of American/non-American literature, how else could you split up fiction? And truth be told, that's generally a hard way to split books - there are often aspects of multiple genres in a work of fiction.

As a result, most libraries around the world have a single dewey code for 'all fiction', then split that by genre, but accept that the genre classification is fluid and open to interpretation - and recommend that patrons check all sections for a specific book.