The efficacy to weight ratio of meat vs. rocks and metal is freakin' absurd. We don't know how to build a robot that's as strong and damage-resistant as a human body and weighs only as much as one. Similarly we don't know how to build something as energy-efficient as a human brain that thinks anywhere near as well. Artificial superintelligence may well be a thing in the coming decades, but it will be profoundly energy-greedy; I fear the first thing it will resolve to do is secure fuel for itself by stealing our energy supplies like out of Superman III.