Many of these treatments will cause needless and unbearable suffering with little to show for it. Doctors have seen, over and over again, patients go through inordinate suffering with abosolutely no beneficial purpose, other than to assuage the feelings of unknowing relatives.
Many of these doctors would absolutely avoid any such treatment for themselves, knowing them to be an exercise in futility.
In life and death emergencies, it has been show that, contrary to common belief, CPR rarely leads to a successful outcome. Instead, it may cause more injury and keep someone alive for only a short period of time while in great agony. Many doctors personally abhor the procedural effects of resuscitation.
Family members are unaware of the undue suffering they may be causing their loved ones, when they give license to doctors to proceed with all measures possible to maintain life.
Here is a link to a very important, thoughtful and well-written article that goes into additional depth about why CPR and other emergency medical treatments may do little good and cause more harm than good, and why such painful and horrific treatments for cancer and other diseases may in fact, border on cruelty.