You're right it'd be sad. But if you climb a tree, you're not gonna reach out to the moon; you're going to be a little out of breath from climbing, and notice your lungs. You'll have put your arms and legs to use. You'll notice something around the tree you didn't. And maybe then you'll do something else.

It's not actually about reaching the moon, but (I think) just doing something. Because purpose and meaning can also come after doing something which previously had no apparent meaning.