[wruza]

Because javascript lacks scope-as-an-object. You can't track `var x; x = value` through it. `setSomething()` sends a notification after an assignment. Also, DOM elements can only take raw values and must be manually updated from your data flow.

Adding these features in-browser would seriously slow down DOM and JS and thus all websites for real. So instead we load megabytes of JS abstraction wrappers and run them in a browser to only simulate the effect.