The only assumption we make when we make comments as such is when we are sure that whatever we've known or is documented in literature is a 100% truth. Paradox is, the only way that the former thought came to my mind is from the experience that I've had or learnt from others' experinces. They themselves might form a very small segment of reality in totality (which again is from what I know or have experienced). I know it sounds crazy but, c'mon worth something to give a thought to.
RE: Science Says We Shouldn't Exist...