You can wrap libtai in almost any language, and that's been done for the bigger ones. Which language are you using, and why is the conclusion not that the language is deficient?
Maybe that's a fun new OS feature. If somebody wants to try it, more power to them.
Answering your question more directly though, why would you want it in an OS? The OS primarily exists to mediate shared resources, and to a slightly lesser degree to sensibly wrap shared code everyone is definitely using (e.g., chrome and hacker news don't have to care about my LCD driver).
What exactly do you gain by baking TAI into the OS? You lose in update availability, OS install size, application-specific customizability, runtime performance of TAI function calls, .... You'd want to gain something for those costs.
> Why would I use a library for a timezone?
Most people do? Even libc localization isn't a part of the OS (and is fraught with issues; never use libc localization), and that's the most primitive timezone library most people use. Everything else is baked into their language runtime or a third-party like nodatime. TAI isn't special in that regard.
