It doesn't assume it. It only assumes a value qualification that is necessary from our perspective: without human morality (which determines the value of things

It doesn't assume it. It only assumes a value qualification that is necessary from our perspective: without human morality (which determines the value of things), there is no guarantee to exist anything that humans value. It doesn't matter if life exists elsewhere if we're the only ones that ever cared about it. Besides, the only alternative is useless optimism - to assume that life elsewhere does exist and implements our values (perhaps better than we do,) and thus we are totally unnecessary and it makes no difference if we all die. (This is the same nihilism inherent in believing in an afterlife.)