Brains are malleable and somewhat self-healing, and not always busy – external stimuli coming through skin, ears, eyes, etc. cause given parts of the brain to become active.
Fact – pound for pound the brain uses a large multiple of the calories used by e.g. red muscle. Resting the brain is a necessary economy. Revving up the brain such as to study, think, plan, design, etc. coincides with the rest of your body going into a resting state. Marathoners may do casual arithmetic about liters of water and miles to go, but they don’t solve quadratic equations in their heads.
Nature / GOD’s design / evolution / what-have-you tends to be very “smart” and economical in just about everything. Just like pumping iron or working out on the track, you can ‘train’ your brain. In fact, learning to ride a bicycle does that. Learning to study does that. Learning to play a musical instrument does that.
The net truth is that our brains are trainable, but not nearly as expansive as we hope.