Your ignorance of laws of physics doesnt change them. Every one with some experience over/underclocking CPU's and GPU's would be well aware of the range and impact of core voltages.
As for why they arent doing it yet; it doesnt make financial sense yet. Hardware prices are still far too high, electricity cost is still utterly marginal for most large customers. Pricing is done per GH, cutting that in half to get better power efficiency doesnt pay off now. Fast forward 6 months and you will see.
What makes you so sure that current gen chips are underclockable to better than advertised efficiency? Like how bitmine advertised low power mode at 0.35w/gh (according to your understanding of physics it should have worked) yet underclocking did diddly squat.
You need some evidence before you can claim so confidently that all current gen chips are underclockable to below 0.4w/gh (at a reasonable $/gh)
Because 20nm should also be cheaper to produce per GH, due to the increased transistor density. Not that I (ever) expect(ed) a 20nm part before late fall, but thats another story.
How exactly is spending 10 million rushing to the smallest node size cheaper? According to nvidia 20nm is less cost effective than 28nm.