I think it's because their boobs aren't worth anything in terms of social capital. I am no prude,  I've done figure modeling for artist, danced topless at hippie events where such things aren't weird, etc, but I wouldn't get anything out of it where others would. It's like I really don't want to see that so I'm not going to participate and encourage it. If I was on a night out at a private party I'd be totally down to dance topkess with other prettty shit ladies. Not with fats though.