"But human intelligence is limited by the size of the skull that houses the brain." When you think about it this way, it seems impossible that we haven't duplicated the capability of the human brain in an airplane hangar somewhere. What's going on inside our heads that we can't mimic? That magical algorithm...
But the _precise_ capabilities of a human brain are not actually what we want.
For starters, an infant's brain does not have any immediately valuable capabilities.
After that brain is exposed for several years to signals propagating through the brain's host body from the surrounding environment, it has developed many interesting capabilities. But those capabilities are only meaningful in the context of the input stream that the brain has learned to interpret.
So if you want your software simulation of a brain, running on a hangar-sized computing cluster, to perform human grade cognition, then you'll have to provide it with a signal as rich as and of the same form as the signal that we receive on a continual basis through our 1 billion sensory cells (optical, auditory, proprioceptive, etc).
And in order to supply that signal in a realistic way, you'll have to simulate the environment in such a way that it responds to motor output from the simulated brain. (Or you can use the real environment, but then you have to have the brain operate a complete synthetic human body).
All this is a tremendous technical challenge, outlandishly expensive and, even when achieved, does not immediately enhance our understanding of how naturally intelligent systems process information. Nor does it provide us with a means to construct specialized intelligent agents that operate in the world, whether autonomous vehicles, burger flippers, surgeons, or stock brokers.