Many times they are elderly and resuscitating them would be more harmful. Others have illnesses where they are not ever going to get better and they are near the end of life in that scenario. Others are afraid of what the treatment of their illness and would rather not seek treatment and when the end comes, to not be resuscitated.
I am in my late 60's. As I get older I have told my family that quality of life is to be considered. If I am in an accident and need to be on a vent short term for recovery, go for it. If I am terrible health and the prognosis is not good, do not go for the heroics.
As a nurse, I have seen families fight the DNR as they don't want the beloved to die. This is all selfish on their part. I have had patients who are at the point of dying and we have to resuscitate or send them to the ED because they won't change the status to DNR. Why? I ask. Grandpa is 88 years old and already in a vegetative state. Understand that we all die and bringing him back is not going to change his state of being alive.