A really complex answer, and several other answers already cover important aspects.
A lot of it has to do with many different levels of economic incentives.
Most increases in life expectancy came from improvements in child survival. That isn’t changing much now.
One factor in that is the increase in organisms that are resistant to the drugs we developed to treat them. That resistance is mostly the result of the economic incentives of drug companies to sell as much drug as they can, so putting them into animal feeds or general purpose cleaning agents, rather than keeping them for treating really important problems. And it is more complex because people often demand their doctors prescribe them the latest drug, even when it is not appropriate. Very few people have a reasonable understanding of complex systems, most oversimplify really complex things to the point of invalidation.
Another factor is around food. The economic incentive is to develop foods with longer shelf lives, this is more profitable for supermarkets, but not so good for people, as our biology is really complex, and we have evolved many subtle dependencies on “micro nutrients” that vary a lot between individuals and are not generally well understood. Such foods are great at providing calories, but not so great at optimizing immune system function. Human health is generally optimized when most food is whole plant based, and fresh (and mostly raw).
Another factor is the need for rapid economic returns, so there are strong incentives to “shortcut” testing systems for new developments. And that is really complex, because it is countered by the absolute need to develop and deploy alternatives to the older systems that no longer function (for all manner of reasons).
Immune system function is inhibited by stress. In these days very few people are free of significant economic stress. Then there are all manner of social and political and technological stressors (nuclear war, climate change, etc). There is no economic incentive at the individual level to remove stress (our news industries are built upon it). Many of our social and economic systems have significant sets of stressors built into them.
We are rapidly approaching the technical capacity to indefinitely extend lifespans, and to provide everyone with a high standard of living based upon many levels of fully automated systems, but there can never be an economic incentive to actually produce such a system, because the very idea of economic value is predicated on scarcity. If you doubt that, just think about the economic value of the air we breath – arguably the single most important thing to any of us, yet of zero economic value. To have economic value, a thing must be scarce to some, and abundant to others – thus enabling trade. When fully automated distributed manufacturing makes most things universally abundant, then there is no economic value. That is not yet a popular or well understood idea. It is very close to being a physical reality.
It is now arguable that the very idea of market value (money) is now the single greatest threat to human life (both individually and collectively), and that is far from simple, as our current economic systems perform many essential and generally poorly understood functions (distributed networks, distributed governance, distributed cognition, distributed risk identification and mitigation, etc – really deeply complex stuff). So transitioning away from market values is a far from trivial exercise, and it is one we must manage, and soon.
So short answer to a long question is – economic factors working against the long term interests of people (individually and collectively). And in today’s world that is a sufficiently heretical statement that it will likely be unavailable to the majority of people who read these words.
[followed by Woke 21 Feb thinking about this question among many others]
How does one determine cause?
What are the many levels of influence present?
How deeply hidden from our consciousness are many of the valence systems that bias us one way or another?
Those seem to be a deeply complex set of questions and answers.