These apps predated LLMs "getting good".

Both have an enormous number of paying customers and you don't just disrupt that for a language change.

Anthropic literally rewrote the whole runtime CC is based on in another language. And even then they still not dared to migrate it away from React.

Zig -> Rust is straightforward and is a good advertisement for their model.

React -> Rust is ambitious and the models aren't there yet.

Soon.