Open the browsers dev tools and you see HTML. Read MDN and you see HTML. Constantly translating back and forth between that and another language isn't worth it.
Though if there was a better language to describe the UI which is as performant, widely supported and documented as HTML, I can easily see people using it. Maybe someone will start that project today based on my comment, and a decade later browsers will integrate it natively because it's such a success and everyone is using it :).
