As one obvious example, evolution is not agentic but still answers "why" questions.

"Why do we have a heart" "Why do we sweat", etc.

But Chesterton's fence is often used in an even MORE generalized way than just that, not "why is it there" but "what are we not seeing about how this connects to everything else"

As an example, eradicating mosquitos. We see many obvious reasons why it might be good, we can even see that they don't seem that important in the food chain, but it would be hubris to assume we understand every potential connection they have to world ecology.

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.