Conversely, if you believe the universe ceases to exist entirely when you die, you believe other people cease to exist too. In this case, perhaps the only consistent way to think of other people is that they are not like you when you are alive either.
From a more down to Earth perspective, if I left the planet on a one-way interstellar FTL journey, confident I would never return and that there was no way to communicate with the people I left behind, even though it would be a relief in some ways and I'd stop caring about people left behind, I'd still want to arrange good things for some of them after my departure. Because I'm nice like that.
Well, you can't be so sure you'll never see them again and you'll never be able to communicate with them, because of scientific advances and such. The only truly irreversible thing that could happen to a human being is death itself. Everything else being reversible (or eventually reversible) is the nice part about being alive.
That doesn’t really work out in a logical sense if you assume that people are purely self interested, but they aren’t, their behavior is optimized by evolution and self interest is only one of many complicated feelings that people have.
I still share your optimism though that we might cure the diseases of aging, so that people do not suffer from slow decline.