...given the questions we typically ask about behavior --- "we" being either psychologists or laymen --- the fact of physiological happenings is typically implicit in the description that starts the question rolling. Though we might not know the details of the physiology, and we might find those details interesting on a personal level, they do nothing to help answer the questions we are asking. Thus, relative to the concerns of the psychologist, physiological facts should rarely be accepted as explanations.And yet, people commonly accept physiological facts as explanations for behavioral or psychological happenings. Why? This question is, I suspect, pretty easy to answer, and the answer is telling about one of the major problems in psychology:
Why do people accept physiological facts as explanations in psychology? Because they are dualists!
I've talked about the weird places dualism gets you a little bit already. When talking about the ending to one of the X-men movies (here), I pointed out that the clever plot twist required us to assume that some mental abilities were intimately related to brain structure, while others were not. Not the worst suspension of disbelief required to enjoy the movie, but the one that twinged my psychologist-nerve.
I think this serves as a natural starting point for some speculation on why people think that information about physiology can explain the interesting features of behavior or any other aspects of psychology. If you think that some psychological happenings exist out there in the immaterial ether, that they are the purview of a non-biological soul, then it makes perfect sense to consider information about physiology to be an explanation of how the body constrains these happenings to create the material phenomenon. If the body is a conduit of the mind, rather than the material of the mind (in part or in full), then indeed the brain could play an important role in explaining behavior.
In contrast, if we were not dualists, then whatever a given psychological phenomenon is, we would already know that it was made of material happenings. Thus, no matter how much information we got about the material that made up the phenomenon, we would know that we were just getting more and more details about the thing to be explained.
Does this seem like a plausible reason for some of the confusion?
On a related note: In other contexts, when I get anywhere near ideas such as this, it is common for someone to assert that psychologists and biologists are far from proving that all psychological phenomenon have a physiological basis. I'm never quite sure how to respond. It occurred to me today that I might reply "Yes, just like chemists are far from proving that ALL chemicals are made up of atomic molecules. However, we have all accepted that the substances we interact with are made up of molecules, and there is more than enough evidence to justify a similar confidence amongst psychologists."
Does that seem like a good response?