So is the computer going to threaten to land the plane at the nearest airport if a bunch of drunken yahoos onboard start pissing on the floor and molesting the female passengers? Will 400 terrified humans who have just dropped 1000' be mollified by a computer voice saying "BE CALM. ALL IS WELL"? When the plane catches fire, do we just let the pax decide which doors to open? How long would you stay on an automated plane that was apparently stuck on the runway for reasons that you don't understand? I spent four hours on one (hot) plane in Denver waiting for thunderstorms to clear, it would have been pretty damn ugly if there wasn't some authority to both coerce us to staying onboard and assuring us (falsely it turned out) that we were leaving soon. Imagine some rumor while flying across the atlantic that the plane is going the wrong way or that communication has been lost...
I have been on automated trains late at night in big cities, it can get a bit scary even for a guy. On the street you can avoid situations that you can't when you are trapped in a little tube, which is not a natural situation for humans.