It is probably past experience with the medical system.
If you have never seen anyone withering away on life support, you are more likely to say do anything to keep grandma alive.
I work in the medical field, I have no interest in being kept alive for an extra week hooked up to machines. I know that when you reach the end, the machines are not going to bring you back. Someone with less experience may believe there is still hope as long as your heart is beating.
I don’t have any numbers but I frequently see advanced medical directives with elderly white people, rarely with blacks and hispanics. That could also be related to access to legal services also.
I had power of attorney for my mother and did not wish to prolong a vegetative state. But I can understand people who’d do anything to keep that spark of life going for another hour or day or week.
I’ve told my kids I’m not interested in being kept alive on machines, but need to put that in writing. I understand that hospitals frequently override DNR orders, probably for fear of being sued.