She didn’t go out for two reasons: They didn’t have a concession speech prepared, and she was screaming and throwing things and was not composed enough to be seen in public.
I agree that her concession speech was probably not prepared.
I only listened to it casually, but I was surprised that it seemed to sound nothing like Hillary’s other speeches... IOW someone else wrote it. Next, she seemed so haughty and stubborn during the campaign that, combined with her alleged propensity for being autocratic, it is difficult for me to imagine that she would have commissioned such a speech in advance. Several hours is about the right amount of time to write such a speech. Maybe Bill wrote it.