The problem is that you encounter edge cases on every drive, and you need to be ready to respond. The car may be able to handle 80% of the trip, but one of those edge cases will sneak up on you and the car. How long would it take you to regain situational awareness and safely maneuver the car in the event of some unexpected situation after you've been cruising in self-driving mode for an hour? Ten seconds? Five? Can you do it in one? What if the car doesn't realize it can't handle the situation at all?

Like any risk, you also need to consider the impact of getting it wrong. If an audio assistant gives you the wrong answer to the population of your hometown, no big deal. But if your car thinks everything is okay and drives you into a stationary fire truck on the shoulder of a freeway when you are travelling at 70mph, the downside of that edge case is infinitely worse.

Sure, humans can make these mistakes, too. But the fact is that your notional world where computers are able to make smarter decisions than humans about how to drive doesn't actually exist. No one has figured out how to make it work. And they won't anytime soon. They've solved all the easy parts. But it turns out there's a lot more involved in driving than all the billions of dollars poured into the problem so far can figure out.