Why do people get old and die?

There is a simple reason: evolutionarily it did not matter if they die at older age.

It the simple rule that things wich are not nessecary will not
dominantly develop in evolution.

Humans live quite long for a mammal, one of the longest lifespans. Only few other mammal species live as long or longer.
(Elephants, Whales etc) Mostlely Animals who have a high
investment to raise their young.

These mammal species specialized in bringing up few young and
extensively invest time in them.

Species who spawn many offsprings (like rodents)
specialized on many offsprings with lower investment in the individual.
These species typically live much shorter.

Now humans probabbly optimized to raise about 4 to 5 offsprings
per woman, starting to reproduce at the age of around 12 to 14. Also twins are few. So humans are a typical 1-offspring species.

Investment in the children had to be at least 8 to 10 years for
it to become independant enough to be a productive member of the group and require less care by the mother.
So the lifespan of women and men up to around 30 years was enough to serve
this reproductive purpose. Its typically the age where people start aging more quickly.

At the harsh conditions back then, most did not make it over 40 anyhow. Life expectancy was much shorter than today.
A few older members where shurely beneficcial (sharing knowledge)
But having a higher percentage of old members would not have benefited the group, and rather slowed it down.
The reproductive and childraising age (for own offsprings) was
over at this time.

There was no evolutionary pressure to keep full health at this age.
So the genes never adapted to support a higher lifespan.
Its not a "killswitch" but rather a lack of optimization to
keep life (the organs and cell) healthy and from deteriorating after such a long time.
It requires costly protection and repair mechanism and thus energy to keep
tissue healthy over long periods.


Image a different scenario: each female can only have one child
in 5 years (because there are 5 year long winter cycles or so)
In this case we would have evolutionarily adapted to live quite longer.
Maybe to keep young and at full health up to 80.

But there was never this pressure.

Nowerdays we live much longer than the original lifespan,
But there the effects of non-repairing tissue kick in.
The aging starts and since we have a much more convenient environment then the club swinging caveman
we can life much longer with pretty overdue organs.