The Kurzweils and Drexlers of the world make a living by claiming that amazing technological advances (immortality, molecular nanotechnology, thinking machines) are far easier than they actually are. When these things fail to get developed in the timescales they predicted, they blame the scientists who failed to make it happen.
Kurzweil has made a living by doing a lot more than just making claims about technology. He's created new and contributed to existing technologies throughout his lifetime, and saying he'll throw up his hands and blame others when his predictions fail to crystalize paints him as a crock and disingenuous -- he strikes me as more than competent and extremely passionate in regards to his work and theories.
His fundamental assumption that change will continue to follow an exponential curve because that's what it's doing now seems highly suspect to me. The problems you have to solve become exponentially more difficult as your tools become exponentially more powerful.
The human mind is the product of billions of years of parallel computation and experimentation. Raw horsepower is probably the easy part.
These two podcasts, http://twit.tv/fib52, http://twit.tv/fib55, of an interview with Rahul Sarpeshkar discussing Ultra Power Bioelectronics provide a good summary of current knowledge on how the brain works. There is a decent understanding of how many areas computationally function. The biggest surprises were that the brain is a hybrid digtal/analog computer and something similar jpeg compression is in operation in the brains visual system
But he's not saying "Because that's what it's doing now", he's saying "because that's what it's always done".
Human / animal muscles are also the product of billions of years of experimentation, and a few decades of steam power research trounced them with ease. Just because it's old doesn't mean it's inherently incredibly good and hard to replicate.
Simple biological systems like bones/muscles evolved long ago and have mostly just been refined since then. Human-like intelligence is a much more recent and much more difficult development.
A lot of people seem to think that if you build a machine of sufficient raw horsepower that intelligence will magically emerge. Maybe, but I suspect it's not nearly so easy.
