Because people are really annoying, but we need to be able to live with each other.

We need strong inhibitions against killing each other-- there are exceptions (self-defense, war), but it's a big win if we can pretty much trust each other not to be deadly.

We'd be a lot more cautious about turning off computers if they could turn us off in response.

None of this is to deny that turning off a computer is temporary and turning off a human isn't. Note that people are more inhibited about destroying computers (though much less so than about killing people) than they are about turning computers off.