Because medical issues tend to be transitory in comparison to the establishment by birth of a human life.
Again, hospitals are not repositories of birth records. Records concerning each birth are maintained by the department of health (or whatever it is called in your jurisdiction). The hospital records are not memorializations of the birth itself, but rather records related to medical care given before, during, and after birth. They are treated no differently than any other medical records, and would almost certianly not be retained after 50+ years.