If loot were awarded based on individual war score, then people would want to maximize their personal scores – which may not be what is needed to win the war. People think they can one-shot, and then fail. They try again with a different team, they fail. Clean-ups may not be glamorous, but they need to be done even if they have a lower potential for racking up points.
Also, defensive is passive. Sure, it took work to collect heroes and troops to have a strong defense, but the selection of the other team between attacking you or your ally may have everything to do with which particular heroes they happen to have – attacking a stronger team may be wiser if your offense better counters the defense. So I’m not sure why rewarding defensive performance would be valuable.