I wouldn't say they never happen in Japan - after all, one of the most sophisticated terror groups/cults in modern history was from Japan (https://en.wikipedia.org/wiki/Aum_Shinrikyo). They were even able to synthesize VX and sarin gas.

I remember being a kid living in Japan, and watching the subway attacks on TV. It was scary.

But to your point, I agree, statistically, Japan is about the safest country there is in terms of human on human violence.