Humans exhibit "intelligence". Humans exist in the physical world. There is some physical process which produces "intelligence". A priori, there is no reason to believe this process cannot be understood, engineered, and re-implemented (whether it be in silicon or in a biological way).

Your argument could have been made for every piece of technology before it existed. Here's what scientists, engineers, and mathematicians do: they either keep trying, or they find a reason why it's impossible.