You're absolutely right that designers, regulators and, in some cases, courts should take every death (and accident/near miss) seriously. But it seems to me like there are two questions here. The first question is, "should we have self-driving cars?" and the second is "how can we make those cars as safe as possible?" When people say, "compare it to a human driver," they are suggesting a way to answer the first question, not the second. And that does not mean that they think the second question is "no big deal."