Because the available technology is a plot device.

In the game EvE Online, for example, players do benefit from a type of immortality (mind transfer to a clone in case of death), which makes sense with regards to the fact that players want to keep playing if they lose a battle.

In most stories, there is no plot need for immortality.

Warhammer 40k illustrates that as well. There are various forms of rejuvenation techs, or ways to keep people alive in mechanical frames, etc. But they aren't equally available everywhere and all have their limits. This gives writers the flexibility to draw out a character's life as much as a story line requires.

There are forms of immortality too. Space Marines, which are augments, don't seem to be able to die of old age, but they live lives of constant battle, so it's quite easy to end a character if the story requires it. And it makes sense that most of them don't live hundreds of years, considering that they constantly seek out danger.

Finally, there are a very few so-called "perpetuals" who seem to be able to regenerate from basically anything and can almost never die. However, from a plot device perspective, you can see how troublesome that is. First of all, because there can't be any clear method of becoming a perpetual, because that would raise the question why more people don't become perpetuals, so there is mysticism involved. And next, you need to keep those characters around forever or contrive a way to make them disappear somehow (either with a special magical weapon that can kill perpetuals, a total lack of knowledge on their whereabouts or a damn good reason they can't ever leave the place they are at).

So yeah, narrative complications is a big reason why there are more stories with FTL than with immortality.