Why? Atheists aren't the only ones who play Minecraft.
Minecraft is a sandbox game, where the activities are up to the player. Experiment with Redstone circuits, build castles, or just plain explore.
I've lost track of how many worlds I've created, worked on, and eventually lost into the void, never to exist again. Here I am, at it again.
So why bother? If, ultimately, my work won't amount to anything, and I won't get to enjoy the fruits of my labor for eternity, why even bother?
Theists know the answer - because of the Joy of the Journey. The end result isn't the point. It's not even the goal, oddly enough. The adventure along the way - the experiences and living, are the point. The point is to live life.
Now, the extent of this philosophy extends beyond the shallow.
This is someone else's castle, in progress, on the same server. We're mostly introverted, keeping to ourselves, but we do rub off on one another. We influence each other - share ideas, increasing the knowledge and quality of each others' lives as a whole.
I can only hope that in my efforts, someone else is encouraged to investigate reality, generate technologies and solutions, derive solutions and cures - that ultimately help everyone.
I have my life to live... and so do others. It adds meaning to my life to help others live theirs.
As a typical sentient being, I fear death, but every time I donate time, energy and money to help others, that fear diminishes. It's like we're being chased by ravenous zombies, and I turn to the others and say, "Go on without me! I'll hold them off", knowing full well that I've sealed my own fate - but I'm fine with that.
I'll continue on, however, in the ripples I've caused in other peoples' lives. That's the only afterlife I need.
Christians already know this. I've just chosen not to throw an invisible sky wizard on top.