Why haven't airlines automated the process of rebooking flights following cancellations and major delays? Needing to talk to a human in order to get a new itinerary confirmed is a huge pain for those flying and probably a big cost for the airlines.

This is actually a thing that works really well in the United mobile app.

I suspect it all falls apart when there are multiple airlines in the itinerary, or when a reasonable rerouting requires different airlines...