A good driver absolutely would have avoided that collision.
Then there are no good drivers. Humans are fallible, even a driver who you might consider "good" will fail sometimes in this situation because of many factors.
The problem with computers is that they don't take into account that people will break the rules and do stupid things, a defensive driver assumes someone will do the dumbest thing possible....
Absolutely not true. Self driving cars are constantly evaluating whether someone is going to break the rules or do something dumb. They do this far better than the average driver, even now. Unlike the vast majority of humans they are indeed keeping an eye on other cars drifting out of their lanes, failing to stop at a stop sign/light, braking abruptly for no apparent reason...
A good driver learns to pick up on these cues.
Sure, but only because you are defining "good driver" as someone who does that. In reality, the number of "good drivers" is very small. Possibly just you and me, and I'm not sure about you.
I don't know of any self-driving software that has deployed such technology, but it's really not as advanced as you think. Detecting whether a car is weaving in its lane and might wander over is not that hard, other cues that every "good driver" might be more difficult, but this is a red herring. Simple defensive driving techniques, e.g. not being right next to another car on the freeway, combined with far superior reaction times make self-driving cars much better in these situations than most drivers are, and the software keeps getting better. Unlike humans.