Comment by pcwalton

It's because of the Gompertz mortality law [1]--the probability of dying at a certain age is exponential. If you assume that the mortality of each age-related terminal illness itself follows the Gompertz law, then eliminating any one of those illnesses won't affect the overall life expectancy much, because exponential growth is so powerful. Even if we had cures for all age-related diseases except, say, Alzheimer's disease, the fact that Alzheimer's disease mortality also follows the Gompertz law (probably a reasonable assumption) would lead to lifespans not dissimilar from our present ones.

Essentially, in order to achieve dramatically longer lifespans, we would need to eliminate, or at least significantly slow, all aging-related causes of death.

[1]: https://en.wikipedia.org/wiki/Gompertz%E2%80%93Makeham_law_o...