Although, "single cosmic ray upset events," are just as devastating.
Beyond specific diseases we understand, it's still mostly a total mystery why we aren't immortal- we have not yet identified what is the basic mechanism of aging, or why it happens at different rates in different species, and mostly our systems are fundamentally capable of repairing and regenerating almost anything, but for some reason get worse and worse at doing so over time. Moreover, this doesn't seem to happen in all organisms- there are many animals that live ~4x human lifespans, and at least one species of jellyfish that is biologically immortal.
> with no medical care [...] something no computer or man made complex machine comes close to.
That's because we get far more units of "work" out of our machines than the person living for 70 years with "no medical care." Some people live just 30 years with no medical care too. And the machine does not need to sleep. We eat food they eat lubrication oil. I don't think this was a good analogy.
> it's still mostly a total mystery why we aren't immortal
While we haven't pinpointed the mechanism, we have a pretty good idea of why, and where in the system we should be looking for the answers.
> but for some reason get worse and worse at doing so over time.
You are a living Ship of Theseus and these "error correction" mechanisms are not perfect. Aside from this there are known genetic disorders which alter the rate at which people age. This is not nearly as mysterious as you're making it out to be.
> there are many animals that live ~4x human lifespans
And what are their resting respiration rates?
> and at least one species of jellyfish that is biologically immortal.
In theory. We haven't found an immortal one yet. They all die. They're also nowhere near our level of biological complexity or capability.