The rule that a person is dead once the brain has died was set out in the 1960s partly so that organs could legally be taken for transplant. A way of defining death was reshaped to fit the needs of a new medical practice.
The line between life and death is not a fact the body announces on its own. Where it falls is settled by history, culture, law and politics, so it can shift, take several forms at once, and be argued over.
Two societies with the same technology and the same expertise can still reach opposite answers on whether a person is dead. One country quickly made brain death routine in hospitals. Another, equally capable, gave it legal standing only decades later, and then only in narrow, disputed cases.
Organ transplants depend on the public agreeing that the donor is already dead. Where people see a brain-dead body as still alive, taking its organs looks like killing, sparking public protest and sharply limiting how many transplants happen.
What looks like the simple march of medical advance is actually a series of contested decisions about what a body, a person and a life are. Impersonal technology forces very old, intimate questions back into the open rather than settling them.