Primarily because smart people don't stop to ask themselves, "of all the things we could do, is this the one with the best chance of making money?"
Graham suggest that most smart people have very ambitious ideas but few really think about what customers want.
I think the problem with many, as with people in their early twenties generally, is that they've been trained their whole lives to jump through predefined hoops. They've spent 15-20 years solving problems other people have set for them. And how much time deciding what problems would be good to solve? Two or three course projects? They're good at solving problems, but bad choosing them.