That works because the power grid and power generators are heavily regulated by the government. Is AWS ready to classify itself as a utility and be under government regulation?
That seems like an unreasonable jump to make. Building power plants doesn't work ONLY because of a regulated energy market, though I'm sure it helps. What if the parent commenter hadn't used a utility as an example? If the example of "economies of scale" were ethanol fuel production, would you ask if AWS was ready to classify itself as a corn field?
Power only became regulated because it became so central to a functioning society.
In a couple more decades cloud computing will be almost ubiquitous and yes, it will become more regulated.
The AWS GovCloud region is already a move in this direction. Also the private cloud they are building for the CIA shows that AWS is willing to jump through various hoops for $$.