I agree that both human intelligence and AI are excellent at optimization. The difference is improvisation. Any AI that optimizes must be programmed to optimize. A robot could be designed with all the same strength and articulation as a human adult. It could be programmed to walk across a room; even avoiding obstacles to find the most efficient path (optimizing). But you then take the same robot to a moderately steep hill (like a wheelchair ramp) and tell it to climb the hill, it would immediately fail. Not because it lacks the mechanics to do so, but because it has never been specifically programmed to make optimizations that account for such an obstacle.
However one could take a young child who had yet to learn to walk and incite him/her to climb the same hill (with candy or whatever) and it will crawl to the top. Even if the child had never before experienced a similar obstacle. Even newborns have basic, uniform reactions to stimuli that they, of course, have never experienced before. It's the basic want/get, need/get, hurt/avoid type of functionality. No programming, no calculation, just naked, primal, reactionary behavior. The ineffable nature of the human animal. The instincts, that all humans are born with, are something that AI can never have. It can be programmed to imitate them, but it can never grow beyond its programming.
The poor understanding of the brain is not due to it's inaccessibility. One could even say that human brains are easier to examine, in any meaningful way, than an animals. A human can engage in a completely fluid and lucid conversation while their skull is cut open and their brain physically manipulated. Electrodes can be inserted and the patient can tell the examiner what his perceptions are. Does he suddenly smell cookies? Does he hear music? Can he speak French? The same physical procedure can be done on an animal, but feedback would be minimal at best.
The idea of the brain being 'mystical' (as in mysterious) is not that far fetched. But I think a better word could be chosen. Perhaps 'metaphysical' would be more appropriate. The nature of human sapience is currently beyond any definable boundaries. Often experts will attempt to weave common human actions or reactions (e.g. kissing, finding beauty, laughter) into other ideas that are themselves only partially understood. I think that even that action, the overwhelming lust to understand and not merely know a thing, is evidence of the human condition.
I think situations like that are where the instinctual nature of biological beings trumps programmatic nature of mechanical ones.