The best way to start finding the answer to that is to stay with the specific. With the specific how of this particular fiction. Specifically, why would the left hemisphere's immediate response to incoming data be to rework it in order to project the illusion of a coherent, rational mind?
Geoffrey Miller is an evolutionary psychologist with another glittering academic background. He points out in his book The Mating Mind that the faculties of the human mind are developed to a degree far higher than you can really account for through the evolutionary pressure to survive.
There's no danger in nature that calls for a mind so powerful it can build skyscrapers, fly to the moon, write epic poetry or split the atom.
Humans have the faculties to do all these things, and yet evolutionary survival cannot account for the emergence of these faculties. Miller points out that there is another, far less well-known, side to the evolutionary coin. Everyone knows about survival of the fittest, but even if you survive for a thousand years, if you never mate, evolution does not happen.
Sexual selection is the shadow child of the evolution family. Sexual selection has a different perspective, but works in fundamentally the same way to survival pressure. As the pressure to survive doesn't really care too much how you do survive, as long as you do, the evolutionary pressure to successfully mate is much the same. It doesn't really care how you mate, as long as you do.
Often, of course, the ability to survive and the ability to reproduce work together — if you survive longer, you probably will mate more. But then, of course, if you mate more, you don't really need to survive that long in order to pass your genes along, and that's what the whole point of evolution is.
Sometimes these elements actually work against each other. The need to survive in a very harsh environment might severely limit the chances of mating (the forced isolation of polar bears is one obvious example). On the other hand, an environment which has abundant resources and few natural predators can give rise to spectacular mating displays that actually hamper the organism's ability to get through the day. A clear example of this would be the iridescent tail of the male peacock — try hiding from a wolf (or even walking around for a while) with that thing stuck to you.
Charles Darwin mentions the peacock specifically in The Origin Of Species as a very clear example of sexual selection at work.
Geoffrey Miller applies these principles to the human mind. His point is this - if, instead of thinking of the mind as a survival tool, you think of it as a courtship tool, allowing humans to build courtship displays to impress each other, you have a much more coherent account of the final product.
Miller's thesis is that the mind is a tool to create courtship displays. Poetry, language, knowledge, buildings, drawings, carts, cars, phones… the whole shebang. Instead of the peacock's tail, you have the human's poem, or the human's ship, or cave painting, or car, or house.
And indeed, many people do have rather impressive houses, and it doesn't seem to hurt their mating chances much.
But then — in Sperry and Gazzaniga's experiment with split-brain patients, we can see quite clearly that the left hemisphere of the brain consistently and specifically creates the illusion of a rational mind, with no hesitation, or any real concern about what's actually going on.
Does it not seem a little strange that fully half of the brain is more concerned with lying about what's going on to project an illusion of rationality, rather than actually being rational? If the brain were primarily a tool to create courtship displays, it would be more concerned with actually creating things, making good things, things to show others.
It's almost as if the mind isn't a tool to create displays. It's almost as if the mind itself is a display. That the brain is a tool that creates the illusion of mind.
The mind as an utter illusion. A striking, and strange thing to say. A new perspective which opens up a different way to look at rationality — and more than this. A perspective that makes crystal clear sense of irrationality, of human madness, frustration, and pain.