good question, i dont know the answer, but ive always wondered this as well. I can only think our sluggish boring modern lifestyles limit our brain useage. in other words, in order to survive in the modern world, we dont need all of our brain to live. maybe the brain is like a graphics card...and theres a way you can get it to overclock! lol
When you think about it though the human brain is still not fully understood, one of the most complex natural machines going, well...i think its "the" most complex one going! Personally, i think, with enough will power, we could get our brains to do whatever we wanted. anything from reading minds, to prediciting the future to the psychotelekinises, only thing is, we cant be arsed, and theres no real need to bother trying! so we might not use our brain to its max, but then, do we really need to?