Because browsers were initially meant for another job: displaying static, declarative documents. Slowly, they gained more and more features and performance unt

Because browsers were initially meant for another job: displaying static, declarative documents. Slowly, they gained more and more features and performance until people started creating single-page apps, where you need to re-render on state changes all the time. So people invented the best ways they could do so within the boundaries of the browser: the diff/patch approach, in order to work around the performance issues of the document model. And in some time, perhaps browsers end up standardizing a way to do this natively. Or perhaps another app-model rather than doc-model (doubtful). They already tried to start with templates and whatnot, somewhat unsuccessfully. In summary: the document model (and browsers) were not designed for that. The Web is a mess of features and features on top of features that has ended up with roughly a secondary operating system on top of a real operating system. The only (but critical) advantage of this model is that it is a de facto standard.