With self driving cars you have no idea what they would do when presented with an edge case they may have only seen very rarely before. With humans, you can in most cases assume the driver would do something reasonable, especially if there is enough time to think through the situation.

You must be driving in much better places than I do. Human drivers do incredibly unreasonable things even without edge cases.

But they understand less and have less common sense by which to operate.

So, it takes a lot more work on the programming side to compensate.