Why is it so hard for homo sapiens to admit when they're wrong?

Have you ever screwed something up in your work or personal life, immediately admitted it and discovered there were no serious consequences… or conversely, made a mistake, refused to take blame even though you and others knew it was your fault and then found yourself in a deeper mess? I ask because of an article I just read about presidents who accept blame (Obama, Chistmas Day lapse in airport security) and ones who don’t (Bush, Katrina). Thoughts?