Why does Blizzard continue with that approach? They add more "waiting for time to pass before you can do something" over which players have no control. This is again part of their new (or should I say rediscovered, enhanced old?) philosophy where they want to extend the time it takes for players to complete content. And this is not by means of making the content more difficult but just enforcing time-gates on it.
Why does Blizzard get away with it? Let me remind you that World of Warcraft is a subscription-based game so it is in their best interest to keep players subscribed as long as possible. Yet such design decisions were not abounding to the extent that they are now. Has Blizzard given up on trying to prolong the lifespan of content by other means than raw time-gating?