The driverless forklift very much does exist and has in some places for years. (I know a guy who works on them,) Fully automated picking is used in some places to great effect. Amazon certainly could build such a system if they decided to do it. The fact that they still use humans says that they have found that to be more efficient/cost effective. For something on that scale, I imagine that it would be more efficient to let humans handle it. Otherwise you would have to have a smart conveyor system that would make UPS look like grade school.