Because these smart vehicles are dumb as fuck when you put them anywhere other than on a well maintained, sunny, first world country highway. Not so long ago teslas braked on the highway when going under signs because they thought the shadow was an obstacle. A few of them crashed in overturned trucks because they though their roof was the sky/a cloud, one even missed a damn firetruck and smacked into it at full speed
When you have thousands of cars out there your one in a billion edge case will happen, this is the perfect example. A ten years old would have braked, the tesla didn't