I suppose it all comes down to beliefs without any form of rational thought.
People believe that their life is the most valuable thing they have, they never even contemplate what comes after, if anything.
Since they believe their life is the most valuable thing they have, they believe your life is also the most valuable thing you have too.
So they believe that they must do everything in their power to extend it for as long as possible, even if you are in absolute agony for how ever long it takes.
Rational though would say - this person is bound to die. This person is in extreme agony. Why prolong that pain and suffering when the outcome is inevitable?