For the rest of us... I guess it kind of boils down to the now-cliche quote "I think, therefore I am." And we usually just accept the axiom that all humans think, and therefore they are, too. Some of us refuse to accept the axiom that all humans think, and suggest that some of them might be literal NPCs, but that's considered "edgy."

Consciousness is the knowledge of being alive. The problem is people think this is supposed to ipso facto change an organism's behavior, when in fact there's no philosophical reason that it should. See the Chinese Room thought experiment.

In fact there is no reason it should exist, given the supposed origin and nature of life as complex chemical processes. Yet it does.