I often wonder this myself, this really should be a standard by now.

I can't speak for the status quo, but for at least the first ~5 years (so until 3 years ago when I last attempted to use it), the JS implementation of Fluent was a mess. Constant issues with incomplete API, wrong TS typings (which at that point were external) and build/bundling issues to the point where we opted for a homebrew solution.

I imagine that I probably wasn't the only one driven away by that (and I gave it many attempts!).

The standard is, for better or worse, gettext; it's good enough that any attempt to replace it runs into the problem that people can't agree on how much better an alternative needs to be to be worth migrating to; so you get a constant churn that so far hasn't seen any clear winner.

Feels like it's That XKCD page; there were standards like gettext, then web development came along and a load of people (...present company included) rediscovered localization and pluralization through trial, error, half-building one's own localization library, then the JS world reinvented it, etc etc etc.