I have thought about it before. You could always fall back on the “realism” aspect that a raiding party doesn’t carry much in the way of supplies to answer this. Of course given the extreme lack of realism in other parts of the game that is kind of a lame reason. But what I really want to win, and you could justify with the “reaslism” argument is recruits.
Note that I’m pretty sure I have read more than once that SG hasn’t implemented an ELO system, and doesn’t want to, so that wouldn’t be my focus on why they should start giving us food and iron on defense wins. Besides I’m pretty sure that ranking is what you win and loose as a defender would satisfy that requirement.