It's pretty weird to me that biological organisms explicitly die. Other stuff can be taken apart and put back together. We can repair our gadgets (although to be fair, we designed them this way). Our bodies (correct me if I'm wrong), on the other hand, begin to self-destruct almost immediately after being deprived of oxygen, which we have to continuously consume to support metabolic functions. And it is conspicuously easy to be deprived of oxygen as a resource even though it's abundant. Imagine if your laptop stopped working forever the first time you turned it off or it ran out of batteries. It would be a terrible design if your laptop was made with longevity in mind, and we typically think that biological lifeforms would evolve to efficiently survive in the long term. We have the innate instinct to survive and prolong our lives at whatever cost (usually), but that seems weirdly at odds with the fact that we're not built to survive indefinitely. Death feels like a bug because of our survival imperatives, but biologically it's a feature. I don't think that it's an unfortunate consequence of entropy or the chaos of the universe, and eventually we all succumb and capitulate to it despite our striving. Instead, with the way we're constructed, it appears like it happens "on purpose". In addition to our own biologically programmed self-annihilation, we also fucking eat each other so... it is definitely built in. Reproduction allows us to collectively extend ourselves indefinitely, but it doesn't solve the underlying tension of our survival imperatives and our mortality on an individual level- maybe because that tension has some evolutionary benefit, and it ought not be solved. Again, a feature, not a bug. Then through reproduction, it's indefinitely repeated and magnified on huge scales of populations.
I get a little too intelligent design-y when I think about stuff like this, because it all seems too overwhelmingly weird to have just happened like this. I think there could be multiple reasons why it happens that way. The least cynical view that I have is that the ease with which we come by death is a mercy. It is explicitly to prevent scenarios like being reanimated and having your consciousness hijacked whenever a mad scientist or a necromancer or whatever coincidental forces happen to reassemble your constituent parts. Can't do that, our bodies nuke ourselves minutes after we die so good luck trying and please don't succeed. This way you know your life is a one and done. You're here, then you're not, and you don't have to worry about waking up in some strange and unfamiliar place after the fact.
The I guess evolutionary reason for me would be that it forces a refinement process. To fulfill your immortality imperative by proxy you have to reproduce and reconfigure genes and that means new lifeforms are made with different characteristics. Maybe this would be energy efficient, lots of permutations of life originating from and being recycled through the same substrate. The tension is instrumental here because, even though on the surface of it it's totally nonsensical, that striving against our mortality and the futility of that striving (as an individual) is what applies the pressure needed to create those permutations. No striving, then no one cares, no one reproduces, the system collapses, everyone dies (happy ending to me, personally). Seeking individual immortality isn't futile? Again, no one cares to reproduce because you can just fulfill your immortality imperative yourself, no need to do the weird proxy reproduction bs (also possibly a good ending). What strikes me is that the aims of the system are accomplished at massive expense of each of the individuals that make up that system. Human society feels like it's just a microcosmic reflection of this...
My more cynical view is that it's a form of behavior control. We basically have a loaded gun pressed against our heads at all times, except instead of a gun it's a peach pit and the head is your windpipe. We have to do things a certain way, otherwise our privilege to exist at all will be revoked on a whim. If we were immortal, we could do whatever we wanted, and no one could coerce us into behaving the way they want us to (except for threatening things like permanent imprisonment). Presently, we can't even survive most of the natural conditions of our own planet (which is something else that I think is extremely fucking bizarre). And by cosmic standards, my understanding is that it is actually a pretty tranquil ass space rock. We can't live underwater. Winters can kill us. The sun destroys our DNA. This forces us to behave in very specific and stereotyped ways. Maybe something or someone out there wants us to, for some reason.
And lastly, my other cynical view is that it's supposed to short circuit meaningful personal development. You grow old and die in such a short timespan that you don't have the time to make anything meaningful out of it (especially when you also have to attend to all of your survival needs at the same time). On a collective level, it encourages evolution, but on the individual level, it prevents evolution from ever getting out of hand. Fearful tyrant gods who don't want their creations to grow to overthrow them perhaps? Keep them repeating the same cycles over and over again. Who knows.
The one impression that I get from all of it though is that none of these forces care about *us*, the actual individuals experiencing all of it (except I guess in the first case). Even if the overall show must go on for some reason (or no reason).