(Sarcasm starts here) Maybe it’s because the fish deemed (safe for raw consumption) are taken from cleaner waters close to Japan and regulated strictly. Perhaps Japanese people’s bodies are immune to higher levels of mercury vs. westerners or other traditionally non-sushi eaters. Hell, maybe the Shinto Gods of the sea bless Japanese fishermen’s nets with a +3 buffer to methylmercury resilience to their catch (gamer speak, sorry). (Sarcasm ended)
For being a country so closely bound to raw fish, especially the kingly Hon-maguro or Tuna – considered a veritable methylmercury sponge (※Note)– and despite being a frequent sushi consumer, I have yet to neither fall ill to mercury related illness nor have borne witness to any local commotion that would suggest it’s a widespread problem here in Japan.