There is evidence that even when things were "done correctly" at Fukushima there were completely unexpected failure modes that no-one had predicted. That's the biggest challenge in engineering safety - handling things that are literally unpredictable.
Earthquakes and tsunamis may not be accurately predictable, but they're not like a meteorite hitting the Earth and wiping out an entire continent or Godzilla coming out of the sea and eating your power plant..
The Japanese tsunami was bigger than the designers had ever expected, but it wasn't (since it happened) impossible. Even if it was the biggest earthquake and tsunami in recorded human history (which I don't believe it was), it was still a genuine possibility.