Because "humanity" isn't really screwed, just most members of humanity. I hate this assumption that climate catastrophe or nuclear war will be a certain end. No it won't. Humans will survive, maybe even a large percentage of them. LIFE GOES ON.
Maybe you want death because trying to improve things is "too hard," but I want to live. I hold onto hope because life is more fulfilling that way. You're just justifying apathy by assuming we're all going to die when it's actually unlikely to happen.