This is in part because too long life may in fact hurt the group. Of course we don’t want to look at that because we are so hell bent on extending life, even talk about immortality.
In nature, dying of old age is incredibly rare as you are much more likely to die from predation or injury. Even if you suffered no ill effects from aging, ie you were biologically immortal, passing on your genes successfully would still require you to have offspring reasonably early in life. There is only selection pressure on your genes up to the point where you have successfully passed on your genes. It makes sense for evolution to keep you alive while raising your kids, and even while helping your offspring raise their kids (see the Grandmother Effect [0]), but eventually the marginal utility of your support drops so low, and the odds you're already dead rises so high, that there is simply no selective pressure. As an illustrative example, you could have a gene that makes you twice as strong as you were at 25, but if it only activates after you're 150, you and evolution would never know.
On top of this, many genes have tradeoffs. If there is any mutation that gives benefits early in life but is harmful later in life, evolution will tend to select for it, especially versus the reverse. Similarly, traits that only provide a benefit for a finite period which is nevertheless long compared to natural lifespan can be favored over traits that work indefinitely (eg many mammals are born with a few sets of teeth and have no way of repairing them). This leads into more general accumulation-of-damage theories of aging.
We see many species in nature, particularly those unlikely to undergo predation and at low risk of injury, with substantially longer or even indefinite lifespans. In the lab, we can also breed organisms like fruit flies to have substantially longer than natural lifespans.
There is no evidence that living longer is harmful to others in hunter gatherer societies of the sort we evolved to live in, and certainly no evidence that such harm would still be present in modern, industrial societies.
[0] https://www.statnews.com/2019/02/22/grandmother-effect-helps...