IMO it's because the web has a huge diversity of behaviors (in a way that, say, native apps do not) but a monoculture on the development side.

React makes sense if you're making Gmail. It doesn't really make sense if you're making a mostly static blog. But because there are more job opportunities in the former (when you consider the wealth of internal web apps out there in the world) all the training courses folks take emphasize React and an app-centric way of thinking about the web.

And perhaps most importantly, it's good enough. It works. Users get by with it. And the developer experience is better than it was in the days of Backbone etc. So few push for change.