Not all doctors can handle emergencies. It requires being able to diagnose and treat in parallel, often times with no information except for what you can immediately gather from the patient in front of you who is actively dying.
Many specialists, when confronted with an emergency involving the organ system in which they specialize, will want "more data" before starting treatment. It's a lofty approach, and the better one in stable patients. However, undifferentiated respiratory distress, hypotension or altered mental status require the approach and skills that differentiate EM trained docs from the rest.
(Of course an FP who has been working in an ER for 20+ years may have these skills, but he or she picked them up on the job, not in residency).