Because they're science fiction? Kind of like asking why we don't just use warp drive for space travel, or transporters instead of airplanes and hyperloops. M

Because they're science fiction? Kind of like asking why we don't just use warp drive for space travel, or transporters instead of airplanes and hyperloops. My less sarcastic answer: Because there's no "we" to decide such things. The companies that own what we're calling AI have little to no incentive to build in the "three laws," should we get to that point, which I doubt. If we had useful robots the military would use them and they wouldn't want robots that won't kill people. If we can imagine robots and AIs with the three laws embedded in them we can also imagine robots and AIs without those prohibitions, and people hacking them. The bad actors will always drive the arms race.