i don't mean this in an offensive way but your question is a little naive. orthopods don't use their general medical training at all, they aren't 'real doctors' but no one ever says you shouldn't go to medical school to become an orthopedist. surgery is a relative newcomer to medicine having been completely separate and a denounced practice by physicians for many years. In the UK orthopods or surgeons are not considered physicians, they are not called "Dr." and in fact would be offended if referred to as such. They readily accept that their field is intellectually bereft and they don't care because they get to play with saws and other toys, identify and fix problems in a technical paradigm, and get renumerated very well for it.
Despite your typical orthopod not being able to manage pneumonia, hypertension, diabetes, AKI, or being able to tell you the basics of acid-base physiology or pharmacology (despite have learnt all this and done pretty well on the boards), orthopedics is paradigmatic of the current medical model. They make an observation of the patient (look), the examine the patient's joints (feel and move), they use radiographic information to facilitate making a diagnosis (x-ray), and based on the diagnosis offer a techincal solution in the form of surgery, or conservative management. The medical training isn't evident in what they know about the vast corpus of all areas of medicine, but in using a particular approach to diseases of the bones and joints, and having a particular approach to intervention. It is their skills, and their professional identity that secures orthopedics as a medical specialty.
In the same way, psychiatry isn't a medical specialty because we prescribe drugs, or can diagnose and treat UTIs or hypertension. It's not a medical specialty because of our technical or procedural skills (since we don't really do any). It's a medical specialty because we have a specific approach to the patients and their problems, that is based in our medical training. We make a careful observation of the patient (appearance and behavior, perception), we then interview the patient (history, speech, mood, affect, thought form, thought content, perception, cognition, insight, judgement), based on which we construct a differential diagnosis, and use laboratory, radiographic and occasionally psychological tests to narrow down the differential (routine and special investigations), and based on what we hear, what we see, what we smell, what we feel, and we experience, we construct a narrative that summarizes and contextualizes the patients problems (psychiatric formulation). This is then used to provide technical solutions and interventions for the patient (risk assessment, capacity assessment, drug therapy, psychotherapy, social and rehabilitative therapies).
Psychiatrists don't have a vast medical knowledge because we're not internists, we don't have amazing procedural skills because we're not surgeons or interventionists, most of us aren't public health physicians so don't have a vast understanding of systems and organizations of care. We have superlative self-reflective, interpersonal and communication skills. Yeah there are many psychiatrists who are terrible communicators, but there are many surgeons who are hacks, and many internists who can't make basic diagnoses. The point is on the whole, we use our reflective, interpersonal and communication skills to make diagnoses, construct formulations, and treat patients. How I relate to patients and how they relate to me is very much in the context of a doctor-patient relationship, and interpersonal and communication skills are a key part of the practice of medicine, it's just much more important for us than for orthopods.
Medical students often have a limited idea of what medicine is about, and given that we still expect you to cram 2 years of mostly irrelevant information which implies that medicine is a 'scientific' enterprise (despite there being nothing scientific about medical school), students often struggle with psychiatry. It doesn't help that there are so many terrible practitioners and numerous approaches to the psychiatric patient. Students often can't understand the idea of feeling one's way into the patient's experience (phenomenology) because they are incapable of doing so. When you find something challenging or difficult, or upsetting as psychiatry can be, and you happen to be a horrendous overachiever, you are liable to dismiss the field as a whole*.
*As has been commented on the basic knowledge needed for psychiatry is easy to grasp and its probably the easiest shelf exam etc. But the clinical skills are amongst the most difficult. at my med school the psychiatry OSCE had the highest failure rate of any exam. Lots of people used to fail the old ABPN oral boards, and the majority of people fail the clinical skill exam of the royal college of psychiatrists. Psychiatry is difficult to practice well, as easy as it is to practice poorly.
...exactly. 
