It’s what you get when writers produce epic book series where each volume’s the size of the Encyclopaedia Brittanica. It’s no different to any author writing a decent sized book, though, it takes a hell of a lot longer to write, edit, re-write, typeset, proofread, correct, check, print, bind and distribute that it does to read the damned book. Terry Pratchett at his most productive had three books on the go at once. Kate Griffin, one of my favourite fantasy authors, only manages one book a year, her two adult Urban Magic books are 450-460 pages, and the third, out next month, will probably be about the same. She manages to write the occasional teen fantasy book in between, and hold down a job as a freelance theatre lighting designer as well. She at least is fairly prolific, managing eleven books in nine years, the first published when she was fourteen…
William Gibson hasn’t managed that in nearly thirty years of writing. Slacker.