Essentially the same reason we need peer review in the first place. Many authors have strong but wrong opinions. But even without malice:Some don't care that the arxiv version is slightly different from the paper.
I dont see why anyone would put different content in the two papers since its so trivial to be ridiculed for that. I dont think arxiv has resources to review if the preprints are the same as the final, and it seems an overkill thing to do .