You can do that if you're building a website, but people want to build desktop-class applications these days, and you can't just sprinkle some JS on server-side rendering and make it behave like that. Also, the same people with a lot of experience in backend work probably aren't that good at creating GUIs, and vice versa.
Why can’t we go back to the old server-side template rendering and use progressive JS for interactivity? Just like the good old days.