People do not pull over to the shoulder when they are between lanes in (presumably) busy traffic, which is where you have to be to hit a median.

I am not sure if Tesla's current system, which is still just cruise control with steering, should ever be changing lanes unprompted. I don't have a Tesla but I would not want my car's cruise control to do that right now. I do think that pulling over in safe conditions is something that a self-driving system needs to be able to do at some point on the path to full autonomy.