What's so damn great about growing old and getting sick and dying after less than a century? The human race makes up reasons why age and death are noble gifts so we won't collapse in despair over the futility of it all.
On the other hand, don't immortals often last less than a normal human lifespan due to head hunting? I get the feeling the century plus ones we see are a tiny percent of survivors. I always had the impression that the life of the average immortal was:
"Wow, I just had my chest crushed in a freak encounter with wheelbarrow full of peach cobbler! Why am I still alive?"
Two weeks later: "Why did that guy with the sword just corner me in an exitless back alley? Too bad my life peddling hot eels in London streets did not teach me anything about sword fighting. These jars filled with pickled fish don't seem help very much at fending him off. In these last moments of my life, I can't help wondering if there is not some larger perspective from which the events of the last few weeks make some sort of sens[squelch]"