I agree, but we're not really talking about simple components on a screen. If that was the case, everybody would be happy to use tcl/tk, or swing or whatever.
Plus, it's not overkill when you get the browser for free.
Plus, it's not overkill when you get the browser for free.
Really, that's the crux of the problem is that there are different paradigms that are not just a little bit different but fundamentally different. This is especially annoying on my Chromebook running Android apps -- they very often expect you to be using a touch-based system without a keyboard. The way this works with a docked Chromebook and a mouse is the mouse is treated as a VERY precise finger touch.
Additionally, on Android and other touch-based systems you're usually not using a Window Manager so, for example, using an Android mail client on my Chromebook, I can't start the reply in a new window so that I can read the message in one window while simultaneously replying... and I especially can't have a bunch of unfinished replies while reading other emails to gather information for the reply.
Creating a meta-layer that can represent the fundamentally different modes from the same data is VERY difficult. HTML and CSS do a terrible job as well unless you model your HTML in a very specific way, which cannot accommodate all GUI semantics.