The way I see it, it's pretty obvious. We have evolved to last long enough to reproduce and set up the next generation in working order. For some animals, like salmon, that's as soon as the eggs are laid. In some spiders the males don't even last beyond the mating. For us, it's long enough to supply the brats with the information to get going.
So anything beyond when the kids hit 20 or so is superfluous from a genetic point of view. Our genes aren't designed (by selection) to keep us healthy thereafter. Because we have also developed memetic transmission, there are some advantages in longer life, but these haven't had time or selective pressure to translate back into genes yet- and in any case, the memetic value might be greater if transmitted by an obviously older person.
As far as the ecosystem is concerned, once we've done our job, the sooner the carbon is recycled the better.
So we're Breaking the Rules by being here at all. Hands up all those who like breaking rules!