Dunno why it's not been/being discussed here, but here's my take:
Very early on in school, kids are taught that you need at least a 'C' grade (70%) to move on, and that getting an 'A' (90%) is the best. The smart kids figure out, somewhere around 1st or 2nd grade, that there really is no appriciable difference between a C and an A, and start tending towards the C because they do enough to get by, and no more. Then around 3rd or 4th, parents and teachers start whining about "you're not working up to your potential." Around 6th or so, you want your parents to leave you alone... so when they whine about your potential, you think that if you do well, they will leave you alone... so you start actually trying. The result? Even MORE attention (praise this time, but you don't care.. you just want them to go away). The effect? Getting good grades didn't get the results that you wanted, just more work.
The net result of this is that during the "formative years" in school, kids were taught that "good enough" is good enough, and never learned to try. When they get out into the work force, they do the same thing. The ones that get out of that rut and succeed instead of fail are the ones that eventually learn that if they apply themselves, they can get out of the rat race faster.
As far as smart people doing dumb things, I think that comes from thinking that you DO know when you don't, and making bad assumptions and acting on them... just like everyone else does.
Disclaimer: My info comes from my experience, and the experience of my friends, not a tax payer funded academic research project.