It's just physics. For a given transistor, more transistors @ higher frequency = more power. At the ultra low end people are using processors that run using energy collected from stray radio waves. While a better design and instruction set help unlike desktop computing the standard is already lean.
I don't know if anyone makes a 45 nm 8 bit cpu. Intel makes 8 bit chips, and has 45 nm and smaller fabs, but I don't the market is there yet.
Still the original Apple II was 1Mhz, with 4kb of ram which is significantly faster and more powerful than many micro controllers in use today. For many solutions the power savings of sub 40khz speeds is still worth it so I don't think we are at the point where stray radio waves could power a system that fast.
