1- As @tgflynn mentioned: historical complexity
The web was made for publishing text, not making rich apps. But it turned out to be exceptionally good for publishing SaaS. So we continued to pigggy back on it. Working directly with the DOM is too verbose, hence nobody wants to touch it.
2- Service oriented architecture
When mobiles became a thing around 2008, normal MVC apps with some AJAX would not cut, because mobile phones needed their own UI. Hence REST based approach became more popular. Things like React and Angular (or Backbone and Knockout) filled that void.
3- UI is an evented runtime
I think of UI as a small Kafka because it needs to handle events (from the user, from the browser window, from OS etc). This makes managing state tricky. To manage this complexity, the community comes up with libraries like Redux. This creates a gap for developers working at a smaller scale. With a small app, you might not run into issues arising due to event management, you might not run into race conditions. But since big companies use a particular library, small companies start using it too. This adds to the bloat.
Is there a fix ?
I think so. But it will include developers thinking outside the browser (or maybe the browser starts thinking outside the DOM?). Libraries like Hyperview and DearImgUI exist and are being used in production. Games are a good example. A game engine can render a complex scene with 100s of moving entities at consumer grade hardware. At the same time websites suffer when rendering large list views.
At the end, FE is a political landscape. A large majority of developers and companies are happy with the current state of tooling. Unless an influential figure pushes for a better UI standard, we will continue to write React.