Currently the software never tells the hardware about the status of the different portions of the memory, for this to work they'd have to add circuitry and kernel support for all this, which consumes power.
These modules are already super efficient, and extra components doing switching banks on/off all the time would diminish much of the power you gain by having them off.

Finally, the RAM is one of the least concerns in terms of battery life, it consumes so little compared to other components like the display and the various radios.