[havill]

If Japan catches you, however, that's an automatic loss of Japanese citizenship. Japan has a specific law that says if you voluntarily acquire (or re-acquire; it doesn't distinguish) a foreign citizenship as an adult you immediately lose your Japanese citizenship.

And if you lose your Japanese citizenship, you go back to being a foreigner. Without a visa. Meaning you risk losing your ability to live and work in Japan.

As most people who naturalize do so because they've established deep for-life roots in Japan, their Japanese citizenship is not something they're willing to gamble.