Why do people who have advanced stage cancer want to fight so very hard to stay alive?

What I am talking about here is advanced stage cancer. You always hear people say, "I am going to fight! I am brave! I am going to go down fighting!" Listen, don't take what I am saying the wrong way. What I am saying is not insensitive. But why don't people just give in to death? Death is natural. Everyone does it. There is no shame in death. Maybe what is waiting for us on the other side is beautiful. When a person is fighting cancer, there is a lot of physical and mental pain. When you decide not to do chemo or radiation therapy, you can enjoy the time you have left on this earth. Of course, there will always be pain along with the cancer, but it won't nearly be as bad as the pain experienced with all of the chemotherapy. And there is definitely no guarantee that chemo and radiation will extend your life. So why not just accept cancer and refuse the treatments and go out of this world naturally? Why not take the leap and go to the other side? The cancer is nature's way of telling a person it is time to go. Why fight nature?