Altered Carbon addresses this, as does Eclipse Phase. Once someone can afford immortality, they go on getting richer and more powerful indefinitely. Compound interest for the win. These immortal oligarchs have an immense influence on societies.
So anagathics.
Traveller doesnt explain how they work, only that they do. So arbitrarily pick what fits what you want, and say well it just does.
Rationalizations and narrative justifications.
Something like, wull, anagathics prevent telomere loss, or stimulate mitochondria function, or whatever, but the human body is such a complex system that this has unbalancing side effects to the point where the body cant take it anymore, and people feel tired, go sleep, and pass away for no discernable reason.
Or
200 years of life with toxin buildup, junk food, stress damage, artilage stress, nutrition deficiencies, radiation exposure, statistical cancer risks, diseases, genetic defects expressing themselves, eventually peoples bodies cant take it anymore. Suicide could be another issue, like a noble with dementia wasting away in an asylum, but lingering on for decades.