Humans being bipedal and "vertical" might have something to do with it too, if the idea is to signal oneself as not being "head-on" it might not be so intuitive

Humans being bipedal and "vertical" might have something to do with it too, if the idea is to signal oneself as not being "head-on" it might not be so intuitive with a human, whose "front" (including the face) is always looking at the dog from above, and whose side is not that distinctive.