Because the kludge of HTML 5, CSS 3 and JavaScript is no match for actuall good UI/UX and platform integration, as proven by all WebApps platforms introduced since the WebRuntime on Symbian OS.
WebAssembly will be the final nail, making the browser just yet another general purpose VM, targeted by all major language eco-systems, using their UI framworks on top of Canvas and WebGL.
Admittedly, I have barely used MS Word in at least a decade, but I find Google Docs a far superior overall experience. Do you honestly have no positive web app experience at all?
Just because browser based OSes haven't been spectacularly successful, doesn't mean that webapps as a whole are failures. I am willing to bet that Google docs has better usage on the desktop webapp than it does on the mobile native app.
WebAssembly will be the final nail, making the browser just yet another general purpose VM, targeted by all major language eco-systems, using their UI framworks on top of Canvas and WebGL.