umm because rape isn't stealing anything? rapists don't rape because they want sex, they do it because they want power.
if you want a word for stealing milk then you'd need a word for every stealing of body material, like stealing bile form bears, stealing genitals for so-called natural medicine, stealing wool, stealing their liver oil, stealing their horns, etc.
what's so different about stealing milk? is it because breasts are sexualized in human society, so you need to compare it with something sexualized such as rape? remember that breasts are only sexualized in humans. animals don't even have engorged breasts/teats unless they are nursing. stealing milk is not in any remote way a form of rape.
btw, before you flame me, know that just because i'm saying it's not rape does not mean that i'm saying it's not bad.