I tend to agree with you that odds are that our brains as purely bio-electric artefacts are enough to explain intelligence and consciousness. I'd like to think so. This could be the case even if we live in a simulation, by the way.
But that we seem to evidence that most of brain function reside in the brain does not mean we can conclusively say that there's no extra element to it that we don't understand.
Note that I am much more comfortable with the idea that it's all basic hardware; the big point here is not that a "soul" or external simulation or whatever is the most likely explanation, but to illustrate just how thoroughly we really have no idea what causes consciousness, or even whether consciousness really is meaningfully a thing. It's a big gaping whole in our understanding of the world that we're just being in the very infancy of trying to attack.
I have a hard time believing that, for example, external simulation would have a “feature” that causes people to remember childhood memories when a tumor grows in a certain part of the brain. A much likely explanation IMHO that memories are really stored directly in the brain.