Fictional battles that are worth to be described in detail might often directly involve the main characters or influential persons and therefore it might be out of question to bombard the battlefield and kill everyone. (wellll... at least as long as you are not G.R.R. Martin and turn everyone and their direwolve into a main character or do not care if a main character dies. When killing main characters, it certainly helps if you have enough of them which brings us back to the first point).