Because no-one is arguing someone will be killed, only that "human drivers kill people too" or "machines will kill fewer, eventually".
Will it be the passenger? The coder for bad code? The person who assembled the hardware for improperly tested malfunctioning sensors? The person who made the malfunctioning sensors?
Who? Because someone is responsible. You can't just spread the blame around and the corporation just pays a fine and shrugs after someone's life is ended.