It's possible that the battles are inserted after the rest of the novel is written. They might be needed for pacing, and as the futures of these characters are already set and written, it's simply easier for no one to die.

It's also possible that the battle is only there as a means of allowing the speeches/thoughts -as exposition - and the loss of a character would distract from this exposition. This may therefore be more important than the 'realistic nature' of a battle.