Because natural selection doesn't optimize for longevity.

What you're saying is true, and it can be helpful to put things into logical terms like that.

But it's a messed up thing when you're a kid. You're thrust into the world, and some people around you make everything alright. Then one of those people leave, forever. As I'm sure many others can attest to, even as an adult, that frustration doesn't go away. I accept that it can be explained as deterioration of the cells, or maybe general entropy, but there's an emotional aspect to it nevertheless.

Somehow, it's comforting for me to remember that it happens "just because."

Humans are extraordinarily longevous. Natural selection doesn't optimize for immortality.

There are biologically immortal animals on earth, so it doesn't exactly optimize against it either.

Which begs the questions "why not?" and "why does natural selection get to say how long we live?".