I'm pretty sure it's not that simple or they would all be advertising/doing it.
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.
If knc could simply lower their voltage why would they spend 10 million on 20nm nre just to get 0.4w/gh at the chip level?
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.