>What's going to stop us from building a human body, including the brain, once we have the manufacturing technology to do so?

it's that we don't know what "manufacturing technology" we need, because we don't have a deep enough understanding of how the brain works. I think the parent's point was that attempts to engineer complex systems fail because these attempts can't reproduce the emergent behavior that arises from many layers of interactions from individual agents. a plane is complex, but it's not a complex system.

edit: I actually disagree a bit with myself - you can make a case that planes have elements of complex systems, but it's not at the same scale as a brain, in my opinion.