But is there an in-universe explanation for the lack of such technology?
Life-extension technologies have a bad reputation. They all either have horrific side effects (e.g. you keep getting younger and younger and younger until your cells implode) or you get zapped back into your early childhood and nobody takes you seriously (in my private canon, the age reversal from "Rascals" was actually permanent and they would have been stuck as children forever; that's what you get for basing your diagnoses on a weekend seminar on speculative genomics). The only other alternative seems to be making some sort of devil's pact with semi-omnipotent creatures from another dimension, which may be more trouble than it's worth.
The only WORKING example of life extension technology would be that creepy guy on DS9 who was pitching the Cellular Entertainment System, a way of seriously prolonging your lifespan by staving off cellular boredom.