As well-defined as Unicode is, surprising that no one has tried to replace ICU with a better mousetrap.

Not to say ICU isn’t a nice bit of engineering. The table builds in particular I recall having some great hacks.

POSIX systems actually have their own approach with "locales" and I it predates Unicode and ICU.

Unfortunately, for a long time, POSIX system were uncommon on desktops, and most Unices do not provide a clean way to extend it from userland (though I believe GNU libc does).