Why Aren't Writers More Inventive?

Why is it that writers are prized for their ability to reproduce the same sort of thing over and over in the same key? Once I discovered the charm and brilliance of J.P. Donleavy's voice in The Beastly Beatitudes of Balthazar B., I expected him to reinvent himself with the same verve in each of his novels. Imagine my disappointment when I learned that his remarkable voice, so hard-won, had ossified ever since he first revealed it in The Ginger Man. (And it's not like the man was content to write his To Kill a Mockingbird and then fall silent; he's written a heckuva lot in the same voice.) Similarly, most short story collections make an author's tics, thinness of experience, or powers of invention glaringly obvious. (I hope this is mostly an illusion -- in other words, that authors are generally more versatile than they appear in print and must appear more homogeneous to satisfy readers and sell books -- but I'm doubtful. Even the most disciplined writer can't help but betray himself or herself; we never kill all our darlings.) You can't read Russell Hoban's Kleinzeit and Riddley Walker without being impressed by how completely he revinvented himself - subject matter, voice, tone, and even orthography are all different. That's what writers should do from book to book, I think. But even Hoban suffers from this problem, as much of his opus is stamped from the same mold.