I have done CPR on people before, and it is astonishingly brutal. To do it correctly, you have to cave their sternum in to be able to apply enough pressure to the heart to actually move blood around. For "Out of Hospital Cardiac Arrest" patients that receive bystander CPR, the survival to discharge is around 10%, give or take. The most common outcome of CPR (if it is successful and you get a pulse back) is days to weeks of dying slowly and painfully in the ICU. The older someone is, or the more health problems they have, the much lower the chance of recovery is.

CPR is absolutely reasonable for a younger person that stands a good chance of walking out of the hospital at the end of it, but 90 pound 90-year-old is extremely unlikely to survive in a meaningful way. It is very reasonable to request to not be put through that massive amount of suffering for a very low chance of any meaningful benefit.

There's also degrees of DNR. There's separate options for CPR, intubation, supportive care, active treatment, palliative care, etc. It's a lot more nuanced than CPR yes/no in most situations.