That's a perfectly valid question.
The reason it currently does not is simple. There can be changes to the map the client might not know about. For instance, another character might *Destruct* the level. This could be handled via curses-style-update-only-what's-been-changed once virtual pad has been established, but it also requires the client buffer all the pertinent information, which just isn't being done yet.
In fact, doing it this way would greately simplify things like animating multi-hued items, projecting missle objects, and allow "record" mode in the client. It's a great idea.
Patches Welcome
Of course, the patch has to work on terminal mode, X mode, Windows mode, and Aqua mode.