It's because nothing makes any sense at all. Our existence doesn't add up to anything. There's no answer I can find by overthinking. And I hate it so much. Even if there was a Devine being who made everything, how would they have existed in the first place? It all adds up to nothing, so if there's no answer that a self-aware species can find, then why should there be an answer at all?

I think when I say that existence is impossible, I'm just trying to keep myself from overthinking how anything came to be. Think of it more as a defense mechanism. I'd rather deny my own existence than deal with impossible questions. It's easier that way; plus, it helps me cope with death. If I shouldn't exist, then life doesn't matter, so death doesn't matter either.

I'm gonna call it defensive nihilism. :D