Most sites use an HTML page to launch a JS-driven app because HTML as a presentation medium is not as responsive and lack the UX elegance that one can achieve with Javascript. It's simply a technology stack that works better at solving the job.
The main problem is that everyone is reinventing the wheel to deliver that kind of experience with custom JS code that augments the DOM in different ways. If browsers and the W3C worked out a way to deliver a similar UX over something standardized that browsers, crawlers and other consumers of the web could rely on, that would get used.
The main problem is that everyone is reinventing the wheel to deliver that kind of experience with custom JS code that augments the DOM in different ways. If browsers and the W3C worked out a way to deliver a similar UX over something standardized that browsers, crawlers and other consumers of the web could rely on, that would get used.