Behaviour can be observed and confirmed. So, by the way, can brain activity, especially in today's era of brain-imaging. No problem with scientifically studying behaviour, and even brain activity. I think psychology is not the study of behaviour and brain. That would be like saying physics is the study of thermometers and pressure gauges.
Psychology is the study of the mind.
Each of us knows the exact question. Rocks and chairs even more probably don't. Since we're not mind-readers, we must hope that it's something about the similarities among those that do have minds, and their differences from those that don't that will tell us. At least it better be, if we are to have any hope of studying and explaining them. Neither can rocks or chairs.