I think most doctors are well aware of the fact that they are not God.
When they give a time estimate, they are taking an "educated guess" based on specific informaiton about this patient, and also based on their past exepriences with other patients. (ie-when Mr.Jones was that color yellow because of his liver cancer, he lived about 2 weeks".)
The stories that "stick" with the people are the ones where the doctor's prognosis was wrong because they seem like miracles. But for every "the doctor was wrong" prognosis story I've heard, I can tell you 20 where the doctor was pretty much right on the money in terms of tiome left.
Yes, occasionally someone surprises us with their will to live, but in the end, if a patient who was told he has 2 weeks to live lives for 17 days, are you going to say the doctor was wrong? Essentially the doctor's point is "not too long so do and say what you have to do and say".