Lacking a belief in a god or gods doesn’t mean that someone is 100% certain about that fact. I call myself an atheist because I don’t believe in a god. I choose not to say I’m agnostic because, as others have pointed out, the word agnostic these days implies being in the middle between theism and atheism and I’m not. I have no reason to believe in any god so I don’t. I don’t believe. I have no reason to believe that aliens are monitoring the human species. I don’t believe that’s happening, but I’m not 100% certain. I don’t feel the need to preface every statement with that caveat.