How would you know what logic dictates? You have none.
It started out that way. Sadly, the content has not been updated to generate sufficient goal posts to keep us engaged.
No, challenge-seekers consume content the slowest because theirs is difficult and takes time to clear. Casuals gather herbs and catch fish and do dailies and are done quickly. In fact, we choose tasks based on our ability to quickly and easily complete them.
I have never failed at anything I have ever attempted to do in World of WarCraft because I do not choose activities that pose potential challenges.