Maybe its because we fear pain. None of us know exactly what we will suffer at the end.
thinforyourself ji very well said indeed. If we were all promised that death will come as sudden, without complications, pain and at a reasonable time (this may be an issue) and that somehow our body will just disappear, I think many will be quite happy with the concept of death. The experience of death of loved ones, attachment, seeing people suffer, and so on create an uncertainty that gets difficult to manage for some, but not all.
St Sri Akal.