Why are some people so stupid?

Read a story today about a bus driver who was stood down because he used a company bus (marked "not in  service") to deliver a large TV to his own home.  Not the crime of the century and probably not the first person who would have done this or something similar, but he is probably the most stupid because he downloaded photos and told all of cyberspace of his deeds on Facebook.  Er, did it not occur to him that this was not only stupid but incriminating evidence.  Then there was the hoon driver who downloaded photos of himself taunting the police after he had been arrested.  He has since apologised and claims that he never meant to disrespect the police and he will never go hooning again!.....and if you believe that, well you probably believe that Billy-Bob really was abducted by aliens and Elvis really does live next door.