Because they practised physic, an old word for medicine, healing. It originally meant ' natural science'. Physics is from Greek physika.. Aristotle wrote a work called that , which we call Aristotle's Physics, which dealt with all natural science. In the C18 physics came to mean what we mean by physics now, There was a time, in the C17, when physics meant a treatise on medicine. Confusing, ain't it ? It seems that the reason for physics becoming separate in meaning, for a specific branch of science, was that scientific knowledge had increased and people saw a need for separate words for specific parts of it. So they invented chemistry for one branch, biology for another and so forth