Best guess is that the process of fixing errata is labor intensive. There's no reason to do stuff like that unless its going to be used.
Updating the PDF copies to properly errat'd versions that will always exist for sale when the print run of the physical books run out to maintain a revenue stream seems like good grounds for "going to be used".
Not to mention updating a PDF isn't all that hard, if in fact those illegal copies Scott had to deal with prior to the real books being release were in fact PDF files sent to the publishers, then typesetting isn't a bother since you aren't printing and just adding characters with the proper installed fonts that are only in house for WotC as they created them.
I don't know what program is used to create these PDFs, but I would hope they are not locked versions so that WotC themselves cannot edit them and add text as needed to pages. What is the Acrobat command? CTRL+T or CTRL+<click> with the Text function selected?
Not real different than using say notepad to edit an ascii file, or wordpad to edit an RTF file.
So time isn't really something that should be costing a lot since you could copy and paste the text from the same location of the errata PDFs.
Heck just add the latest errata pages after the final page of the PDFs. Not as professional as doing it the right way, but at least it is included since you don't expect to print out your own PHB, Adventurer's Vault, etc from PDFs as that would cost more than the physical copy.
Working with and fixing a PDF is not as hard as some would have you believe, it is just the manner in which it is done and by whom. If, like Scott suggests, a new person is needed to be hired for that department, then I think that should be done quickly as WotC does a lot dealing with text that it might improve product quality and speed of delivery with less stress involved on all parties within and without the company.