If I were a doctor, I couldn't imagine making a blanket statement that everyone would want the same thing. Whether or not the patient is healthy, in the vast majority of circumstances, you're patient will have family members, and if anything goes wrong, they WILL want to know why. No matter the life expectancy.
Life saving, and life threatening decisions always need impeccable documentation of the circumstances. What if the patient doesn't believe you and wants a second decision? Obviously a second opinion isn't an option in trauma and emergency scenarios... but it's a touchy subject of willful opportunism to decide to take advantage of trauma situations, when you have to prove that you didn't decide to take advantage of emergencies for personal gain.
Or maybe they WANT to suffer and die. If you interfere with a persons wishes, and anything unexpected happens, it risks going before a judge.
Anyway, even if you save someone's life to day, and yeah, it extends their life for six months but with a far FAR more miserable death... well... is that an unsuable position? Lawyers will argue anything.