A very simple hypothesis as to why doctors can argue this effectively but teachers cannot is that teachers are not as powerful a lobbying group as doctors.
Doctors have effectively been able to defend their "turf," from hostile encroachment, while teachers have not, not because the situations do not contain substantial parallels, but rather because doctors are politically strong while teachers are politically weak.
Doctors have effectively been able to defend their "turf," from hostile encroachment, while teachers have not, not because the situations do not contain substantial parallels, but rather because doctors are politically strong while teachers are politically weak.