The browser was intended to render documents, not applications. In particular, HTML has a tree structure, which means that things that are semantically related

The browser was intended to render documents, not applications. In particular, HTML has a tree structure, which means that things that are semantically related on the page and update together are often miles away from each other in the tree. And the page in the browser is part HTML, part CSS, part Javascript. Frameworks try to let the developer work in JS only and generate the rest. And finally, much of the application's state is often kept in a backend, and access to it a asynchronous. I think that's why Web development is so distinct from other UIs.