Because I've seen it quoted, misquoted, and usually not attributed at all… “Converting PDF to XML is a bit like converting hamburgers into cows." That is the quote I know of. It comes from revered XML developer Michael Kay on the xml-dev mailing list in July 2006.
It's possible Kay got this from somewhere else, but I've never seen an earlier attribution. (Comments are open if I'm wrong.)
I hear all sorts of chest-beating about attribution in data circles, often for good and sufficient reason. I think we can stand to get our quotes and their authors right.