system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are bodies able to create an entire body with it's own lifetime supply of regenetive cells, but is itself unable to prevent gradual decay over a lifetime?
gpt
It's a lot like a car.

A new car you want to repair and replace as much as you can. As the car ages, it gets more expensive and time consuming to repair, and easier to just replace the whole thing.

Now consider that evolution is about what survives and prospers as a species. And consider the innumerable dangers that could destroy an organism. Nature has a few creatures that do effectively live forever, but as a general rule, replacing constantly works better, so that is what prospered and spread.

Also note your own body does the same thing. Rather than make cells that never die, it constantly removed and replaces cells. It's such a good system that some of your cells even self destruct or kill other cells to make this happen.

Further, note that replacement allows evolution to happen faster. The shorter a creature lives before procreation and the less it sticks around after, the more changes will happen.

Edit: to summarize, it's possible, but evolutionarily speaking, not desirable. Creatures actually moved away from that as time went on.