Because the authors of these works decided to highlight other possible problems, and not the problem of immortality / longevity. But there are also works on this subject, mostly posthumanistic. Sterling's "Schizmatrix", his "Sacred Fire", "Altered Carbon" (the series is not very good, perhaps the book is better). Dune is also about this in a sense, at least the later books, starting with the third
Apparently technology has gone so far that people began to pump cars more than their organisms, so as soon as I see a new film about the future, I think that all the people in it are cyborgs (It is better, of course, not to think about it, to worry about a the car is heavier than a dull bag of bones)
Because science fiction is not written according to logic, but to make it interesting and pleasant for the reader. And the general reader, as a rule, wants everything in the future with his body to remain as it is now. However, it is not a fact that people of the distant future will live much longer than we do today and at the same time remain young. It is more likely that old age will lengthen, that is, say, 90 people will look like they are now 55.