Isn't it cute that your friends see it better to use their magical powers to beat you at Monopoly than win millions of dollars on the casino roulette wheel?
Why would our brains evolve in such a way that 90% of it is wasted? Does anyone ever stop and think about how stupid a thing to say that is? Seriously, we are the products of evolution and evolution doesn't slowly develop massive abilities which sit dormant. It's as absurd as speculating that we have gills and wings and haven't yet noticed.
Brains are very complex, which makes them difficult for us to understand well. People often start speculating in a 'religious' sort of way when there are big unknowns (like with religion itself in response to where the world came from and why the sun does its thing and all that).
If you stop having a 'religious speculation' approach it's easy to understand the potential of the mind. It's the same with arms and legs. Why can I have a reasonably healthy, functional body, but not win a gold medal in gymnastics, archery, weight lifting or sprinting? It's not because the other guys magically unlocked mysterious gifts which I haven't stumbled across in my own body, it's that they worked bloody hard to develop their bodies and/or talents and in most cases had genetic predisposition for those abilities while I drank beer and went to uni and played with lizards.
Same with the brain. There's no mystical switch you can flick to turn on some ability which evolution decided to hide from us (seriously, do people actually think that or are they taking the piss?). Some of us are born with brains better at some tasks rather than others (you may be creative and musical, I might be better with figures and logic, someone else might be very practical, etc.). On top of that, some of us work hard to develop our minds, some sit around talking crap and watching movies. We aren't all born with the same potential, that's just crap they tell people to make the stupid ones feel better. Results come from actual work or even just luck, not magic, unless you're in a Disney movie.
Seriously, this sounds like trying to find a way to make the Easter bunny bring you more chocolate or wondering why the bunny gives some people more chocolate than ever.