What does "actually in control" mean? When the IRA set up a checkpoint in Rosslea, should that have moved the border on the map? When there are disputes over uninhabited islands, do you want it to show whoever visited first? Obviously these are small examples, but the same issues apply to full-scale civil wars.
You'd also be essentially legitimising the right of conquest if as soon as country A invades country B you move the border on the map. That would be pretty counterproductive.
