Why can't browsers use a virtual dom internally as an optimisation?
Virtual DOM is something like a workaround for the current situation. The W3C are working on re-building the DOM and make the current "virtual DOM" native to the browser. But you know how slow this goes - it has to be drafted, talked over, accepted, and then starts the fun part - implementing it in different browsers. They still have issues with CSS3 and flexbox model - every browser has it's own terms and standards for working with those.
And it's the same with the Virtual DOM - they still haven't accepted it to be a cross-browser solution. This will eventually happen in the future, but until then - we use the JS option.
If you follow the JS world - it was pretty much the same with Promises - we got the bluebird and jQuery implementations, but at the end Promises went native and all those libraries and frameworks are no longer needed.