[OkayPhysicist]

That's easy: because literally everything else is contingent on it. Either we survive and flourish allowing any of our other actions to matter, or we doom ourselves to extinction, in which case literally nothing we do matters.

[idlewords]

We're already all individually doomed to extinction, yet this doesn't prevent most of us from finding meaning in our lives. And in the long run, any civilization, no matter how flourishing, is doomed. Conditioning whether anything we do matters on the existence of an infinite chain of future progeny is a losing game.

[natch]

>infinite chain of future progeny is a losing game.

There's no need to take it to infinite extremes. There are always plenty of values in the slider between short-medium-long term to choose from to find meaning in life… whatever floats someone's boat.