Things that might seem theoretically easy according to present science, with the right tech, might not actually be so easy, or at least not messy. We might not know what we don't know, or once we do have the tech to go down those avenues of investigation, they might not pan out like we'd hope - or they might develop differently than we would imagine.
As it is, we know in Trek that humans have a bad history with genetic manipulation/betterment of the species, so that would ethically close off that line for them. Also, they seem to have done away with most of the diseases that presently shorten our lives, and improved old age generally through various means. I'd say life has been extended quite a bit in Trek. Just because they haven't achieved immortality yet doesn't mean they won't, or haven't tried; nor do present ideas of trans-humanism have to mean much to the society hundreds of years from now.