I think they use it simply because it works. There could be a girl who's an 8 walking around next to two 10's and she looks like a 4 and the 10's don't look much better. but there could be an 8 walking around in between two fat 3's (the highest they can reasonably go,) and even while fanning her face and looking like the stench of fats make her sick, she'd still look like a 10, until isolated from the fats and only surrounded by humans.