Humans can create tractors, and those tractos will have purposes unto us. The tractor will perform a function and that means the tractor has some purpose in itself. But that doesn’t mean WE have a purpose. The tractor doesn’t give US a purpose. It is unidirectional impartation of purpose. Only something that has created us can give us a purpose. Just like God creates us and gives us purpose, but our existence doesn’t give God’s existence a purpose. That would be nonsensical.
And we don’t need to have a purpose to be able to give a tractor a purpose, because the tractor will simply serve a purpose to ourselves, and then the line will stop. The tractor doesn’t have any ultimate purpose beyond its service to us. If the tractor wasn’t created by anyone, it would be purposeless.
See, meaning isn’t derived from meaning. One thing doesn’t need to have meaning, strictly speaking, in order to be able to give another thing a meaning. We see that in the case of the tractor. Meaning is derived from creation, intent, purpose, authority. God has the authority to give us meaning and that’s the only way we can have any ultimate, genuine meaning.
And yes, if there is no God, once we’re dead, it won’t matter. That’s precisely it. Life is absolutely meaningless because of this fact, and there’s nothing you can say or do to ultimately make it any more meaningful. Everything you do in life will ultimately be for nothing, because it will all pass away. All of your aspirations to make a difference in the world amount to nothing, because the world and everybody in it will pass away, and everything will be forgotten. The universe doesn’t care what you do.
The scarcity of life doesn’t make life more precious any more than the scarcity of cystic fibrosis makes it something worth holding onto. In your worldview there is no God; your life is worth nothing. And a life that is worth nothing will be worth nothing no matter how fleeting its pointless existence is. I hope your life is fantastic, because if your life, like those of so many, is full of poverty, pain, and heartache, you have nothing to look forward to.