http://www.uesp.net/wiki/Lore:The_Secrets_of_Ragnvald
This suggests that Otar was corrupted by 'dark voices,' changing Otar from a 'good' to a 'craven priest.' Possibly this is linked to the effect of the mask? Maybe the masks, considered 'gifts' by common understanding, were a method the dragons used to corrupt and control the priests? We know that they were mostly peaceful to their subjects, and then became more cruel, so it could be suggested that the masks had something to do with it, no?
More a role-playing thing than anything in terms of game physics though, obviously.
Don't forget that it is still a game and therefore needs to be balanced. There is no point in having "godly" artifacts (gameplay-wise) because they will destroy the fun. I agree that some of the uniques have enchantments you can find on leveled loot but do not forget the sheer number of artifacts with really unique effects e.g. Dawnbreaker, Wabbajack or Konahrik (which is a really unique dragon priest mask)