This stupid phenomenon is due to the fact that Japanese Gov decided to teach only arbitrarily 1000 kanji to school kids and this number decrease every 10 years.
https://ja.wikipedia.org/wiki/%E5%B8%B8%E7%94%A8%E6%BC%A2%E5...
You can see how stupid it is.
> People complain that kanji being used in the prefecture are not include in the list (no included so no obligation to teach in school) e.g.阪鹿奈岡熊梨阜埼茨栃媛, so the gov finally add those to the next revision.
While at the same time Chinese people are learning 3X more. No one ever complain about the difficulty of Chinese character after all.
It's effectively "all kanji" as it were, and yet Taiwan has one of the highest literacy rates in the world, and I never met any Taiwanese when I lived there (for years) that complained that Chinese was too difficult.
Japanese has onyomi and kunyomi. The onyomi also come from different periods in Chinese so there's multiple onyomi for most Kanji.
Then you get two Kanji words that come in all varieties. Most are onyomi + onyomi, but you get some that are onyomi + kunyomi or kunyomi + kunyomi or kunyomi + onyomi.
There's also not really any solid rules to it, and when there are, there are plenty of exceptions.
It's a real nightmare of a system. A fun one though.
And how many years does it take to? What about words that you have never seen before? What about ambiguous or uncommon readings that require furigana even for fully educated adults?
As I said in the sibling comment, I live in Japan and every Japanese person I have met complaints about the massive effort it took them to learn how to read and write.