Why do people get old and die?
There is a simple reason: evolutionarily it did not matter if they die at older age.
The reason is not so simple: A species with no aging, in combination with other factors such as being not too low in the food chain, had a huge evolutionary advantage. It could grow much more rapidly. In fact it is absolutely not clear why evolution has not developed immortality of most higher species.
A plausible theory is that a non-aging species would not be able to evolutionary evolve further, and thus are stuck in a primitive state and in evolutionary niches.
Yes, for a species still being (virtually or theoretically) immortal there's probably no evolutionary benefit to dying off more quickly. There's either enough natural selection to take away enough individuals from the gene pool for a steady rate of evolution or there is very little evolution going on to begin with, perhaps because of some kind of isolation.
It's usually wrong to look at the dying aspect of individuals as some kind of trait for an entire species that's supposed to be inevitable. Evolution itself is never some kind of goal in itself.
If a species is able to reproduce many offspring that in their turn can reproduce quite quickly, dying more quickly becomes beneficial to evolution, but only because the new individuals do not have to compete with the older generations as strongly when it comes to the food supply.
On a cellular level, death easily makes far more sense. In order to renew or 'fix' things, you will usually need a mechanic that is able to remove damaged cells. Cells that are able to live forever in their turn will constantly be renewed or the actual structures will become ever weaker.
I'd assume the origin of mortality actually lies in the ability to repair ourselves on a cellular level.
