It isn't used because it's just not how the suppliers of content nor consumers of the web want to use the web.
If they want a full featured app, they use it. Even with super fast speeds, latency and minimal startup time still make the browser unattractive for this.
Web users and content providers want linkable documents.
I'd argue that the modern browser stack is so good that we see installed apps adopting html/css/js for the presentation layer.