Here are the factors contributing to the Japanese amateur ranks, especially around 1d, being so inflated.
- Nihon Kiin is controlled by professional Go players for the professional players.
- As such, credibility of amateur ranks is not a high priority for Nihon Kiin, but pro welfare is.
- Every Japanese professional player is entitled by Nihon Kiin (or, Kansai Kiin, etc.) to issue amateur rank certificates for a fee.
- As such, rank certs, mainly 1d certificate (because price x demand is the largest there), are source of pro income.
As a natural consequence, inflation and regional rank/strength disparity (the more the number of pros in the region, the weaker 1d’s are in the area) became ridiculously large, and most of the serious Go players in Japan are quite aware of this situation.
So kyu/dan system in Japan mostly has lost its meaning, and people rely on other methods, such as Points Rating System (https://senseis.xmp.net/?PointsRatingSystem) and its variations, to find good opponents to play. I was a bit surprised to find the Go Salon in Nihon Kiin HQ building using this Points Rating System several years ago thinking “Hey, this is like their own admission that kyu/dan system is not working”.