They probably don't want to do it because then everyone will complain that the battery life is garbage. My specced out 16 GB MBP barely gets 4 hours without being plugged in.
Why aren't hardware/motherboard vendors using RAM hotplug to power down some of the RAM when unused and save that battery life? OS's support RAM hotplug already because it's needed for server use cases, the one missing bit is hardware support.
