Is Google maps a document? Obviously not. But a document can still embed a Google map meaningfully and sensibly.
For comparison, consider that apple just proposed adding a html tag to display 3D models, just like an image.
The problem is that even basic interaction with a 3d model requires complex controls. There is no single standard for materials and lighting. The going methods are optimized for photorealism.
So adding a model tag just means baking in a set of assumptions, and doing so before you have a good idea of the kinds of things people will want to do with it and can't.
This is the real story of the web: poorly conceived standards and APIs, frozen before the medium had time to find its feet. It's a horse designed by committee in a world of cars.
You could turn the question around: why should webpages be dynamic at all? Just make a dumb client that requests rendered SVGs from a server after a client tells you its screen size.
Oh but that can't work because it's not really a document, but a dynamically laid out program after all.
If you want to displace the single page apps of the world, design a "document" format actually capturing all the nuances people want from them... Or get off the pot.