Driverless cars do not know how to read humans and anticipate what human drivers will do. Humans know as they enter an intersection to begin to look for clues about what the other human drivers are doing, or likely to do so we can turn safely. Self-driving cars aren't this smart (yet) as they cannot read body language. At some point, self-driving cars will begin to communicate with each other at intersections so they can safely turn left.
Source: https://apps.bostonglobe.com/business/graphics/2017/04/driverless/series/teaching-a-driverless-car-to-turn-left/.