Because if your family member is violently killed by a robot car doing something that seems inexplicable to the human mind, and with a fact pattern that doesn't match up with normal human error accidents you're used to, you're not likely to be consoled with a reminder that four people you've never met supposedly are still alive because they weren't killed in some place you've never been by a human driver that never took the wheel.
People accept that humans are imperfect and cause accidents. Accepting that computers will make mistakes and kill people is another story. Especially if the owner of the car is a 1 percenter or a mega corporation and it kills an innocent bystander.
Also, people aren't that accepting of human imperfection and expect to see people have licences taken away and jailed where they've been deemed to be behaving recklessly (even if they've racked up huge numbers of miles with no previous incidents)