Because Bethesda never patch their games.....sure they tend to get most of the absolutely gamebreaking bugs (Then again, Vampire Cure quest on PS3 was never patched)....but most of the minor stuff goes by the wayside.
Its been this way since Morrowind, doubt they'll start caring anytime soon....I think it's got something to do with having already gotten your money

Hence why there is always a huge bugfix complication mod for Bethesda games, to fix all these obvious glaring issues.