The human body evolved over time through natural selection. Until very recent advances in medicine which have yet to affect evolution, the entire body depends on various components (heart, lung, brains, etc.), and if any one of them fails, the whole fails. Thus it made no sense to evolve a brain that would noticeably outlast the heart, or a heart that would outlast the lungs. Any effort in a longer-lived brain would be moot as the heart would fail first, and similarly, any attempt to have a heart that would beat longer than the lungs could breathe would be over-engineering. The marginal rate of return of extending the life of any critical organ would probably be equalized in such a scenario.
A similar logic has been alleged to apply to autos, with planned obsolescence. Why design a frame that outlasts the engine? The ideal, from a narrow efficiency point of view, is for all parts to fail simultaneously with no point in spending money on repairs, and no excess wasted at the outset by having parts last longer than the whole.
Both the human body and technological artifacts like automobiles are finite systems. While the date of reckoning for bodies or cars may not be known in advance, nobody naturally lives past about 120 and intensively used cars do not economically (or typically) last past 20 years.
But there are other systems that are potentially infinite. These include cities and networks. While a city or network may not last forever, its potential lifespan is quite uncertain (we may assume that if the sun dies out and planet earth is abandoned, cities will be too, but the 5,000,000,000 year upper limit is beyond comprehension). These potentially infinite systems last significantly longer than their component artifacts. Just as Heraclitus said 'no one ever steps into the same river twice', one never steps into the same city twice, it is continuously evolving, as parts are abandoned, destroyed, replaced, or rebuilt. Quite often, the city while changing its buildings, maintains its networks, whose topographical (and topological) structure outlasts its buildings, in part due to property ownership regulations. (The example of London being rebuilt using essentially the same routes after the 1666 fire being the most illustrative of this case).
I argue the reason that cities (and their networks) last longer is that their components fail at different rates. If all of the components (buildings, plexus (networks, social structures)) failed at the same time (e.g. a fire plus a breakdown of the legal system ensuring property rights), then the site could be abandoned. But as long as most components last, a few failing will not destroy the city. The resources from the remaining components can help rebuild the failed ones. Similarly, networks do not fail together, and the failure of one link (given some redundancy) will not cause the network to collapse.
This volatility in failure rates of components leads to a more stable whole. The price is that only piecemeal, rather than systematic overhaul of the system is permitted.