Probably because it's so improbable. It essentially assumes that we'll hit the limits of technological innovation without discovering that any of our current ideas about physics and the nature of the universe are completely wrong.<p>Long before we could ever make such a device I suspect we'll discover something along the lines of zero-point energy being real harvestable, that the universe is not in fact material and local, etc. I'm sure there are thousands more possibilities. And any one of them being true would make this device completely obsolete way before our technology advances to a point where we can create it.