Board Thread:Naruto Discussions/@comment-12509038-20140806073433/@comment-17623-20140807114737

They probably aren't. Both Kakashi and Obito only had one eye each, therefore they probably used a greatly reduced amount of chakra. Had Obito or Kakashi had both and was doing the dimension wormholes that almost killed Obito in Operation: Save Sasuke's Ass, the eyes probably would have gone blind years ago.

Now that Kakashi has two and is able to bring out Susanoo, it stands to reason that he can go blind like normal.

All this is speculation though. A likely reason could just be "Story"