Native GUIs are fast, but they are not powerful. You have to specify a fixed resolution, fixed size, and more often than not calculate the layout yourself, or use very limited auto-layout features.

The web browser gives you a full package with standardized commands to control all those aspects, which is also portable between different implementations.

If your rendering needs are limited to a few buttons and a canvas on a fixed size, sure, go native GUI. But if you need to support multiple devices and resolutions, on-the-fly resizing, and layout of multiple complex screen sections, the browser is an unbeatable platform.