something that replaces humans doesn’t need to be 99.9999% reliable, it just has to be better than the humans it replaces.

But to be accepted by people, it has to be better than humans in the specific ways that humans are good at things. And less bad than humans in the ways that they're bad at things.

When automated solutions fail in strange alien ways, it understandably freaks people out. Nobody wants to worry about if a car will suddenly serve into oncoming traffic because of a sensor malfunction. Comparing incidents-per-miles-driven might make sense from a utilitarian perspective, just isn't good enough for humans to accept replacement tech psychologically, so we do have to chase those 9s until they can handle all the edge cases at least as well as humans.