In the early 1960s sci-fi writer Fredric Brown
wrote a series of short stories under the umbrella title 'Great Lost Discoveries'. The three he completed related the tales of how invisibility
, and invulnerability
were discovered, and quickly thereafter lost by unexpected quirks of fate.
In 'Great Lost Discoveries 3', Brown makes the observation that, when people say that they wish to be immortal, they really mean that they wish to be eternally young. The protagonist of Brown's story develops an immortality serum, but holds off using it until he is terminally ill; after taking the serum he finds that his disease has become immortal too. He lapses into a coma from which he never wakes, and after a few decades his pragmatic doctors simply bury him.
To live forever but not be eternally young would be hell - eventually your body would be an immobile mass of cancer, and your mind would be a senile wreck. Transferring your thoughts into a computer would not be a complete solution, as you yourself personally would still die; an exact duplicate of yourself would have all the fun.
In the end, we all die because nature did not need to make our bodies immortal; given that we can reproduce by our teenage years, and that we only need to be parents until our own children can breed, any extra life beyond the age of 30 is a waste. And natural selection abhors waste.