You'd be surprised how many have been carefully OCRed and proofread. Many older pirated titles are better than what WotC provided for folks to buy. This has less to do with title popularity or value, and more to do with the uploader's prestige within his circle of "friends" -- quality and attention to detail are well-regarded.
This is interesting. So are you saying that pirates, due to proofreading and correcting the typos, actually provided a superior product? That's actually a pretty shocking revelation to me and it is a major problem for WotC. I suppose on the positive side it may make them invest more money into proofreading so as not have a lower quality product that the pirates can 'improve'. I guess that is a bit off topic for this thread, but it is interesting to think about.