Because they are autodidacts.  The main purpose of higher education
  and making all the smartest kids from one school come together with
  all the smartest kids from other schools, recursively, is to show every
  smart kid everywhere that they are not the smartest kid around, that
  no matter how smart they are, they are not equally smart at everything
  even though they were just that to begin with, and there will therefore
  always be smarter kids, if nothing else, than at something other than
  they are smart at.  If you take a smart kid out of this system, reward
  him with lots of money that he could never make otherwise, reward him
  with control over machines that journalists are morbidly afraid of and
  make the entire population fear second-hand, and prevent him from ever
  meeting smarter people than himself, he will have no recourse but to
  believe that he /is/ smarter than everybody else.  Educate him properly
  and force him to reach the point of intellectual exhaustion and failure
  where there is no other route to success than to ask for help, and he
  will gain a profound respect for other people.  Many programmers act
  like they are morbidly afraid of being discovered to be less smart than
  they think they are, and many of them respond with extreme hostility on
  Usenet precisely because they get a glimpse of their own limitations.
  To people whose entire life has been about being in control, loss of
  control is actually a very good reason to panic.