I don't see the difference. Why do you think a general-purpose computer can't be built which is equivalent to a human brain? Not strictly true. Evolution is about adaptability, not efficiency. Evolution doesn't create the best, perfect, smartest thing. It creates the thing which continues to exist. [Citation needed]. You also seem to presume artificial computers will always be silicon-based. There's no reason we won't use carbon and hydrogen in the future, if we find out it's better. In fact, Intel just stated their 7nm generation will require something besides silicon. I think the bigger question is parallelism. The brain more closely resembles a neural network (which computes differently than an ordinary processor), and a massively parallel system. But we've been moving in that direction for several decades. We hit the heat/power wall in the mid 90's, and everything has been about parallelism since. Not just the "dual core" and "quad core" that consumers see. Internally, individual instructions are being made far more parallel (the technical term is ILP – instruction level parallelism). To be clear, I'm not one of those futurists who think we'll have Sentient AI by 2020 and the Singularity is Here! But I don't think technology is going to stop progressing, and I'm always skeptical of claims "it can't be done."The interactions and chains within the human brain will likely be always more efficient and better than those of machines which are not specialized.
If they weren't better than what we make computer from, we wouldn't be made of them.
Carbon and hydrogen do a billion times better