But I'm talking ten steps back- why do people even seem to exist? Why does any being find itself in Samsara? Why would any aspect of existence ever develop any perception of duality in an existence that is said to truly be non-dualistic? Why is there anything other than pure, monist, perfect enlightened awareness in existence?
If I get the gist of what you are getting at now I'll try to offer some better thoughts. I think the question is why, if the world is Unity, does the sense of separation and suffering even exist then. It has to do with how we developed as a species for certain evolutionary advantages.
In the way back machine we would see ourselves undifferentiated from the world in our self-senses. We were an immediate part of the ecosystem functioning on instinct and impulse as any other evolved life on this planet. But something happened in our brains that allowed us to begin to differentiate ourselves from the world and from each other in a way that created a world of duality on a much more immediately conscious level. We saw the world mentally in me/not me terms. It became a world of subject/object duality as we began operating more dominantly with our conscious minds rather than strictly impulse/instinct.
Doing so offers us clear advantages as a species, but it also creates issues in the system that we now have to try to deal with and correct. We are essentially monkeys with over-sized brains, and that sense of natural awareness that keeps us alive out in the ecosystem, interferes in our structured worlds created through our mental abilities. We are a neurotic species. Our higher evolved brains don't know what to do with all that animal energy in this sack of skin we're in now.
Some have found through mystical experience we find that Peace that brings us into harmony with ourselves and the world. They assume naturally that we 'lost' that harmony when we became humans, and so our myths typically speak of a fall from paradise, a time when we lived with God, then something happened to expel us from that unity. But in reality, in evolutionary terms, there never was a state of realized unity in our past. What there was was an undifferentiated fusion with the earth, with the universe. That is very different than a differentiated unity with the world.
An infant is undifferentiated from his mothers body after birth. It has no emergent self-sense. But gradually in time that differentiation into a separate self begins to occur in stages. He bites his hand then bites the blanket. "Me, not me" begins to surface. But that 'me' is identified exclusively with the body. It is not an egoic me. That happens later, and as that happens it moves through stages of differentiation, 'me and mine', becomes his immediate family group, his peer group, his community, his nation, etc in ever-widening circles of self-identification, trying as it were to "reunite" with himself with what seems lost within himself, having been thrust from the womb out into the world.
But what this, this paradise lost, is a way to symbolize what is happening within ourselves as we reach, not backward to an undifferentiated state (which in evolutionary terms would be a devolution, or movement towards death), but towards a realized Unity with the World, or God. He wants to go back to the womb, but cannot for that way is barred by death. He has to go forward instead into the Unknown. He doesn't want to cease to be, but to know consciousness Oneness with the womb that gave us birth. Not simply crawl back into it in an unconscious slumber. We as humans differentiate, and the find another and marry with them, the 'two become one' as it were. We are seeking marriage. We are seeking to evolve our differentiation from the world, into a differentiated unity with the world. That is the nondual experience, the unity of formless and form, not an undifferentiated, unconscious fusion with it.
Why does evolution, or life move us in this direction? Now isn't that the Heart of the Mystery itself?
I hope I did a better job of addressing your question this time. If not, well they're interesting thoughts anyway.