Followup thought: It's quite possible the answer is "it shouldn't be so problematic, but working in theory doesn't mean it applies well to real-world humans." The answer could be as simple as "something about the process draws out corruption."

I'm trying to figure out how much of the bias against it relates to the bias against self-publishing, which used to be a valid concern, and now we have a notable difference between self-pub and prey-on-authors vanity press publishers.