Because when you're coming in on runway 17L at DFW, all nice and programmed into your autoland, and suddenly the controller tells you to switch to landing on 17C, you have to just fly the plane. If you muck about trying to reprogram your autoland you'll be halfway to Oklahoma before you're set up again. See the video I linked in another comment in this thread for more details and examples of why complete automation is undesirable.
There were incidents in the past where pilots didn't have enough experience flying the aircraft manually. For that reason, most airlines prefer if pilots land manually as often as possible so that they can do it in case the system fails. If you always land automatically, pilots lose situational awareness.
