Minecraft was grown out of the way Notch works.

What you are suggesting is impossible. No one, not even Steve Jobs sits down and says "My life mission is to benefit humanity". This is because we really do not know which efforts will have the largest impact on humanity. You could waste a lifetime trying to make a better battery - but it might never happen.

Or you could hack around on a game and watch it explode and impact millions of lives around the world.

In my opinion the important part is that you are doing something meaningful to you as an individual. If it ends up exploding and benefiting humanity all the better. But the world would be so much better off if we just had everyone doing this.