It wouldn't suprise me to learn that it's basically planned obsolescence - i.e. that there's an evolutionary advantage to a species that cycles through individuals as rapidly as possibly (while leaving enough time to produce a lot of progeny).
EX: A fruit fly needs far less cancer defenses than a squirrel. And, 2 fruit flies can become 2,000 fruit fly's a lot faster than 2 squirrels can become 2,000 squirrels. But, fruit fly's can't eat nuts so as long as trees produce nuts there is a nitch it takes something with at least the mass of a squirrels to fill.
Iron and heavy metals accumulate in tissues over time. The tissue membranes accumulate an increasing share of polyunsaturated lipids, which in turn interfere with efficient metabolism. Many ways that biological tissue wears out over time are well understood. It's not just some mystery.
That's an unwarranted claim. Various quantities of interest of complex systems more often than not follow certain statistical distributions. This is especially true about natural systems.
Flip 100 coins, call it one trial. Perform such trials a large number of times, a few hundred let's say. For each trial count the number of heads. In the end create a histogram (frequency of N-head trials, N ranging from 0 to 100). It'll look like a gaussian distribution! The emergence of such distribution feels strange, even though we pretty much know everything about the system. (Incidentally this is related to the Central Limit Theorem). Similar idea applies to human mortality distributions.
The best you could say is that these are very counter-intuitive observations/facts.