By the time we've figured out how to live significantly longer, I reckon we'll understand "why" we don't. That's kind of a precursor in this case.

But, I should be clear, I don't believe there's any reason to believe the answer is "because we're supposed to die". There is no "supposed to" in evolution, no right or wrong, no ethics, only survival. It is merely a series of improbable occurrences that led us to this point, and I see no reason to attribute moral intention to the result.

Every argument for death, absent a religious decree, comes down to "because everyone who has ever lived has eventually died, usually painfully" so it must be correct because everyone does it, even though most of those folks would have rather not.

And, the reason we don't is almost certainly mundane; we aren't needed after procreation, according to evolution. But, I think humans still have value after they have procreated.