It's been said that in real life, the first person to live to 150 has already been born. And that by the time that person reaches 150, the first 1000 year old will have already been born. Our own medical technology is advancing pretty steadily, and at this rate, immortality will be our b*tch. There's no good reason as to why Sci-Fi like Star Wars doesn't already have immortals. So the answer is: plot convenience.