Humans are actually great drivers when the situation requires thinking. Lot of snow and can't see the lane markers? Millions of people adapt every day to this during winter. Lots of pedestrians, bicycles, motorcycles, etc. doing somewhat unpredictable things? Again, look at any Asian mega city, people can adapt very fast. Basically, if the situation requires being alert, people are very good at driving.
When are people bad a driving? Whenever it's monotonous. Bumper to bumper traffic or a free flowing freeway. Constant repetition of red/green cycles while going down a suburban street. These boring situations make a lot of people basically turn off their alert thinking. Then they do other things like texting, talking on phone, etc.
And these boring situations are exactly where AI self driving is better at driving than people. Computers never get bored. The self driving car will be at 100% attention even during the most boring traffic. But when boring suddenly turns to not boring? The current state of AI is very very bad at this.
And unfortunately there's no easy way to use the best of both sides. If the AI is fully driving, then it will do great while the driving is boring. But by the time the AI decides, this is too much, can't handle this, it's too late to alert the human driver to take over. But if the human driver is required to always pay attention, what's the point of the self driving car?
Self driving cars will get there someday, but I think it's much farther away than many assume it will be.