Because we live to make an impact on this world. We know death will arrive. So we must simply take action. We must find purpose, as we are not born with one. We have to make our lives worth something, and a mindset of "we will die so it doesn't matter" is a simple and inane mindset. Everyone is aware of that. But some things do matter. That's why we live. To do something, even the littlest thing, in this world.