Because it's useless save for a single kind of development (websites) and the main issue is not even in spitting out WASM but making the language runtime and libraries work in the browser. Most languages have no first-party WASM or Typescript support either.
You are right, Apple has no interest in supporting Javascript, what good would it do them except more work and hassle?
It would help many programmers though, to make their code more portable and available on more platforms. It would not be that easy to do, but also not that difficult. Rust is supporting WASM, Scala is supporting Javascript. Kotlin as well. Apple can't do it? Come on.
Just stating the obvious: The interests of Apple and programmers are aligned in some aspects, and opposed in others.