Being relatively low level is a feature. HTML / CSS / JS are languages, not frameworks. One liner modals are not a feature a language should have.

If you want, you can make a framework on top of it that will behave like a desktop UI framework with standardized elements and behaviours. There are plenty of such frameworks actually. But not having to use such a framework is nice.

Besides, the current browser environment has been historically heavily optimized for websites, not desktop apps. If it became a popular choice for desktop app UI we'd have developed more appropriate features for that use case.