Perhaps because, even if it were possible, eternal life -- even eternal youth -- would become unbearably tedious. What would be the point of developing a potion that would result in soul-wrenching boredom, even if it were possible?
Robert Heinlein pursued this idea in his Time Enough for Love in which Lazarus Long's lifetime was extended by many centuries (I think he had lived for almost 1,000 years, but it's a long time since I read the book and I could be wrong). Long became so bored with his life that he decided to end it himself.
Of course, medical science has gone a long way to extending our life expectancy. A boy baby born in 1850 had an average life expectancy of 38.3 years, a girl could expect to live to 40.5. By 2011 those figures had improved to 76.3 and 81.1, so we're working on it.
Here's a graph you might find interesting: Life Expectancy Chart.
