This is a perfectly reasonable question, and I think there are two aspects to it.

First, one of the research questions tested by Xilem is whether it is practical to write UI in Rust. It's plausible that scripting languages do end up being better, but we don't really know that until we've explored the question more deeply. And there are other very interesting explorations of this question, including Dioxus and Leptos.

Second, even if scripting languages do turn out to be better at expressing UI design and interaction (something I find plausible, though not yet settled), it's very compelling to have a high performance UI engine under a scriptable layer. I've done some experiments with Python bindings, and I think in an alternate universe you'd have apps like ComfyUI as high performance desktop app rather than a web page. Also, the layering of Xilem as the reactive layer, backed by Masonry as the widgets, is explicitly designed to be amenable to scripting, though to my knowledge there hasn't been a lot of actual work on this.