Perhaps due to the fact that automatic face detection is an invasion of privacy.
The fundamental problem is that a camera is a privacy removing tool. If someone takes your photo, now they are in charge of your privacy.
Right, we are back to the days when taking ones photo was like stealing their life.
Why is automatic face detection an invasion of privacy? Isn't sharing the picture in the first place the invasion of privacy? Facebook is making it possible, but your friends are the ones doing it.
Now that you mention that, that's a good reason. Would it be possible for facebook to use this to build a database of photos of people's faces or facial features? I really don't like the sounds of that.
I apologize -- I really don't use any social media personally outside twitter on occasion.