These apps predated LLMs "getting good".
replyBoth 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.
replyZig -> Rust is straightforward and is a good advertisement for their model.
replyReact -> Rust is ambitious and the models aren't there yet.
Soon.