Mostly it's the science of how the two systems function. The nervous system is more adaptable at up/down regulating transmitters/receivers in the neural junctions which doesn't require a lot of energy. The muscular system needs to literally grow, which is somewhat even against biology. Big muscles are often not only unnecessary, they're only really made when they're forced to be made, and if you stop lifting, they'll go away. The nervous system will down-regulate too given the chance but its a faster responding system.