We have reason to believe it's possible because we know we can simulate a single neuron. It's just the sheer complexity of full brain we can't scan and efficiently simulate.

There's absolutely no reason to believe that the brain can be reduced to a collection of neurons. It's what we want to believe, because it's simpler and makes us feel in control; but in real nature there's intelligence without neurons, and the brain needs much more than neurons to do its thing.

P.S. If we presume that human intelligence is a random processes, then there's no reason to think that this random process can create another that has more information complexity than the original. It makes no sense mathematically.