1) For most of history, humans and their ancestors didn't live very long. When you die at 30, there's not much selection for biological mechanisms to fix the problems that happen in someone who is 60. Natural selection is looking for genes that are good at reproducing themselves, not genes that are good at giving an individual long life. Once you have sexual reproduction in place, it is much simpler to just have lots of offspring than to evolve even more repair mechanisms to extend life (at no great benefit to genes).
2) That's the whole point of SENS; repairing the damage for which we DON'T have repair mechanisms because of point #1. For example, our lysosomes (the "garbage collector/incinerator" in our cells) accumulate some molecules that they can't break down because they don't have the right enzymes, so they accumulate throughout our lives and end up affection cell function all around our bodies (lysosomes are full and can't do their jobs anymore). SENS has found other organisms that do have these enzymes, and are looking at ways for us humans to use those enzymes to clean up that "long-lived garbage".
> Cell damage is allowed to happen.
Yes, because your genes don't care about you (so to speak - read Dawkins's The Selfish Gene). After you've reproduced, there's no pressure to fix damage that only affects you after reproduction age.
Nothing causes more suffering in the world than the diseases of aging, and curing those diseases should be priority #1. Living longer is just a welcome side-effect.
