we will never get to the point of replicating human ability and energy efficiency with the materials that exist in Earth

That is flatly incorrect. There is a type of ai that is literally just replicating the human mind, hardware and all. That is well within our current technology, although the connections would not be the same as it would merely be a clone. But a cloned human is an artificial intelligence.

I know that form of ai is not what you are referring to, but why not? What is it about ai that makes it impossible to replicate in a metallic substrate? And even assuming a metallic substrate is flatly impossible, that still doesnt stop progress. There are youtubers currently working of making an artifical rat brain in a jar play doom. This is not a piece of a living rat, these are rat neurons grown from stem cells that were converted from skin cells. So we could just as easily start progress down physical ai's.

As for evolution, that was millions of years of random chance, the difference between that and guided evolution is too great to even compare. And the materials came from Earth in the first place. The entire idea of ai is based around replicating what nature did in the first place, thats how all our technologies are made. People said it was impossible for people to fly merely a decade before the wright brothers. The only difference now is that there is no material scientist on earth that claims wed have to go to outer space to replicate the hardware neccassary for ai.

Edit: forgot about the first part of your comment. this should largely cover that