The underlying reason why anthropology needs to have a strict code of ethics more than physics does is because anthropology involves dealing with human beings. Physics and chemistry tend to involve research on inanimate objects while anthropology involves doing research on human beings.
One thing about anthropology is that anthropological researchers are almost always in a position of greater power than their subjects. Western, modern anthropologists have a great deal of power in the world when compared to members of Amazon tribes, for example. This means that anthropologists are in a position where they can harm their research subjects or, at the very least, have a great impact on them. This is not the case in chemistry or physics, which means that a code of ethics is more necessary in anthropology.
Second, the fact that the subjects of anthropological research are human means that they have human dignity that must be preserved. This is not true of research subjects in chemistry and physics. Let us imagine that an anthropologist is studying homeless people. The anthropologist has to behave in ways that recognize the inherent dignity of the homeless people. In order to ensure that this happens, a strict code of ethics is needed. This is not necessary when dealing with chemicals or with subatomic particles.
Thus, because anthropology deals with people and chemistry and physics do not, anthropologists need a strict code of ethics.