As Dr Steve Novella memorably put it (paraphrased), you never hear anyone say, “the good news is that he was shot in the 90% of the brain we don’t use.”
The truth is that we use the entire thing, but not necessarily at the same time. Bits become more or less active as you do this and that, but over the course of normal life you use all of it. The brain uses up far too high a portion of your energy budget to be allowed to go unused. Natural selection won’t allow that.