> Why is it unrealistic to expect a single doctor to have mastery in every medical specialty?

Full stack doesn't mean everything.

And actually all the doctors are doctors first, so they know everything about medicine, except they are not specialized in all the medical specialties.

Which is rarely a requirement in medicine.

Brain surgery or conjoined twins separation isn't that common, but an highly specialized surgeon is perfectly capable of curing common diseases just like a cardiologist can perform common surgery, such as appendicitis.

Full Stack means that you can get things up and running, not that you know everything of everything.