Writing almost anything with javascript sucks ass to begin with. Writing native code with it is even more painful. Most javascript engines don't even have any decent model of parallelism. It takes zero imagination to see the problem here
However, I think it's true that controlling threading in order to perform gnarly work in a separate thread is more ergonomic on mobile than in JS/React.
You'd need to create a Promise that wraps a Web Worker, which would be an unusual thing to use. I don't think most apps need such control over threading in the browser.
yeah because they don't care about a top notch user experience.