What was that question again? Correct me if I am wrong .. was it "Why doesn't he use artificial body parts instead of actual dead people?"
First the shield: I don't advocate, support, condone or enjoy Witkin's work, or his methods. Period. I would not make images like that.
I would answer the initial question with a couple of others... "Why doesn't anyone/ everyone use mannequins for figure studies, instead of real models? Or, why do some trek through the roughest of wildernesses to photograph a mountain, when we could, with much less effort and expense, make a model of it, and photograph that?
WHY Witkin does what he does - his motivation - is another question, entirely.
There are infinite possibilities... one that I can remember from somewhere, is that he shares the perceived Mexican fascination with death and the dead.
Whether that is the motive or not, I don't know. Perhaps anyone searching for answers could search the web for a Witkin site... and it might be possible that he makes a defining statement about his motivation there. I have not done that, because frankly, I don't care.
One thing that is ticking me off here are those that make the assumption that whoever does not make the approved choice in their aesthetic appreciation - whatever that may be - is automatically "devoid of all moral fibre". That patently makes no sense.
My opinion .. We are all free to make our choices in art! If we try to restrict or coerce others in their choices, we will, of necessity, confer on others the right to restrict our own.
I would suggest that aesthetics and morality are intrinsically different subjects, Different, but not necessarily exclusive.
Would it not be proper to start a morality thread? There we can discuss the principles of right and wrong ... or are the myriads of comparative religion sites on the web enough?