Components that are not used, that almost always get replaced by vendors do not get much love. That the base AOSP should be optimized to use out of the box is a purist fantasy. In reality AOSP is made with the understanding that vendors are going to customize it. A lot of Android is designed to be modular, you can easily install a different keyboard app.
CJK languages need language-specific AI conversion engines for any kind of typing. Which OSS versions exist for Japanese, I don't know for Chineses(CN/HK/TW - IIUC they're slightly divergent beyond fonts).