A psychiatrist teaching a psychology course is unconventional, and that lack of convention itself will make it difficult for it to happen.
Some of the lower level courses could be taught by trained monkeys
Several courses I took in undergrad were taught by teaching assistants who were people still working on their master's degrees, so there is some truth to that, and again as mentioned above, that's no criticism of psychology itself. Several courses in college no matter the field are taught by TAs.
Here is how it is unconventional. Typically, a psychology professor does not make as much as a psychiatrist. The psychiatrist who takes a psychology professor's job will not make as much, and consider that psychiatrists graduate with debt often times about double that of a Ph.D. psychologist. Psychiatrists are taught things with a clinical & medical model in mind. Psychologists aren't. Psychologists are also taught the behavioral sciences in more of a pure model vs a medical model. E.g. we psychiatrists are taught the about mental illness etiology, method of diagnosis, labs & treatment, while psychologists learn everything about the behavioral sciences including ergonomics, workplace politics, workplace efficiency, sensory & perception etc, regardless of mental illness or not.
In college, while a psychology major, I learned that sunlight may affect the hypothalamus in a manner that causes females to enter puberty sooner vs later. That was never taught in medical school, nor any psychiatric curriculum I've ever seen.
Psychiatrists also think about things differently. E.g. they are heavily medically biased which does change outlook. E.g. there is no medication which has an FDA approval for the bonding effects of oxytocin, and few doctors I know of even know that oxytocin has that benefit. If teaching a psychopharmacology course, would a psychiatrist teach this given that the effects of oxytocin in the behavioral sciences is not taught in a psychiatric curriculum?
For the benefit of psychology students, you need to teach the material in a manner that is compatible with their own needs & desires. E.g. they have to eventually take psychology boards. Several things in a psychiatric curriculum are not tested on their boards. It'd be unfair to them to teach them a course that does not cover what they need to know, & from what I understand would make that course not accreditted. In fact I have not read the guidelines that allow for a course to be accreditted, and that may include requiring a psychology degree of some sort.
If you become a psychiatrist, and really have the bug to teach, you can still do so in a medical school, residency or to other medical professions such as nursing. If you want to teach in psychology, make sure its kosher.