Requires close-tab to serialise the state of *every* JS context associated with the tab, and JS was not designed to support state serialisation in quite that fashion. Essentially it'd mean closing tabs didn't free memory for quite some time which would be risky.
These days, browsers have to treat pages as independent applications. Essentially it's up to those applications to support some sort of recall-draft if their inputs ought to support it. I suppose you could write an addon which uses local storage to do that kind of thing though.