Conceptually we long debated unifying everything and in an ideal perfect world we would have done it. The reason was one primary goal for a new standard was to make it easily parseable and having unique rather than repeated codes was key to that. Sadly in the end we did not unify everything only to get buy-in from all major countries to support. That’s even why you see the roman/asciii characters repeated within Unicode itself—like as romaji. This was all well in good until we came to CJK and the number characters with semantical overlap was huge that this was more seriously considered—infact we started investigating this at Xerox before even thinking about Unicode and that work predated and influenced and leveraged the work done later.
Japanese users are angry when system chooses Chinese fonts over Japanese, Chinese users apparently suffer with a font mishmash of Chinese and Japanese all the time like a mid-word capitalization. The official sanctioned solution is to just commit a genocide and nuke the offending language out of the system or to attach IVS to every Japanese characters which is like zero padding every single characters by a byte or two.
It's really putting everyone in pain and making absolutely no one happy, which sounds like what a good compromise tends to be, but LLMs seem to be struggling with there being multiple completely separate language families sharing codepoints but not character shapes or syntax. I've seen a smaller LLM model use made up verbs spilling over from the other language. I believe ML guys don't like datasets that do that kind of things. Same apply to image models.