Why Don't Servers Self-Consolidate?

With the mad rush to virtualize and consolidate servers, there's quite a market for "Consolidation planning" tools. (i.e. PlateSpin's PowerRecon, Provment's Capacity Planner, and others). But what I don' t get is that, once you've figured out how to optimize your consolidation -- like a "best fit" for puzzle pieces -- things will change. So, a few months down the line, most datacenters have to re-consolidate.

With all of the automation tools becoming available, you'd think that some of them (I have one in mind) ought to be able to continuously monitor the active services, priorities, resource needs and available resources... and then continuously shuffle-around the virtualized applications to ensure "best-fit" consolidation. That way, you never need more than a maximum number of machines for any given service demand level. And, if you're really smart, these automation tools ought to physically power-down the unused servers to save on power & cooling.