[scarmig]

UIs in general are pretty hard unless you have a set of primitives that do everything you want. HTML + CSS used to be lacking (and still are in some respects). A thin layer of vanilla js on top of them has a tendency to turn into a mess of spaghetti as more and more features are added. So, you organize it into a framework. But a framework needs to be able to assert control over anything that would affect how it renders, which is pretty much everything a browser might do, leading to an SPA.