Whether you could or couldn't self study to be a competent physics teacher is besides the point if you need a physics or very closely related STEM degree plus test of content competency as a condition of getting the job by the regulations if most US states. That limits the pool of applicants to people who can command better salaries elsewhere over their whole career arc for arguably a lot less stress. Thus an exceptional shortage within that specialty is more prone to happen. Lowering the standards of qualification is an option, but the author doesn't seem thrilled with that idea. So we are pretty much left with discussing a pay differential within the profession, which is a third rail topic among most teacher unions.