Generic GUI toolkits are very often worse than the DOM. I've worked with Swing, MFC, SWT, Android, and iOS, and all of them are significantly more complicated than the DOM is. They do function at a higher level of abstraction, and the two really nice things they give you are components and a sane layout system (solved by WebComponents and Flexbox, now if only all the major browser vendors would just adopt WebComponents...), but in general I've found that iOS development is about 1.5x-2x slower than web development and native Android development is about 3x slower, roughly on par with Swing or SWT. MFC was a clusterfuck, I wouldn't even go there.

The way frontend devs for other platforms deal with it is to specialize in the nuances of the platform and effectively build your app around the capabilities that Google/Apple/Microsoft/Sun provide. That's fine, and makes for a fairly uniform, polished experience if you have good devs, but it significantly restricts the set of apps you can build. The reason web developers like developing for the web is because it doesn't make you a sharecropper in Google/Apple/Microsoft's world, while building a full-featured GUI framework into the browser would.