It's laziness. Instead of recognizing that everyone got where they are now by a different path where they learned different things and appreciating the diversity of knowledge that creates, it's so much easier to say, you should have known that.
The latter actually requires conscious effort.
Like: "there's a tiger behind the bushes!". Not everyone knows that, since the tiger is difficult to spot. Everyone however knows what a tiger looks like, and what danger or source of food it represents. So, anyone who doesn't understand that sentence is an idiot.
Conversely, if someone who tells you that stars are Balls of Gas Burning Billions Miles away[2] is plain crazy… unless it's your which doctor, of course. That one knows ways you don't.
Except that the last two paragraphs are now bullshit. A few centuries ago (very recently, by evolutionary standards), our world became more complicated, and our instincts haven't quite caught up yet.
Laziness? Lack of empathy? Yeah, sure. But the effort required to expect the inferential distances we actually have to deal with is very real. Heck, I wouldn't be surprised if that last paragraph fell short for anyone who isn't familiar with the LessWrong Sequences[3]. Sorry.
[1]: http://wiki.lesswrong.com/wiki/Inferential_distance
Computer people are notorious for having trouble explaining technical issues to computer-illiterates for this exact reason. It's not that their unempathetic - they are trying to explain so that the conversant understands, after all - but rather, they've been experts for so long, they forget which technical terms the conversant should or shouldn't know, or what does and doesn't need to be turned into an analogy. Overcoming this is a difficult skill.
In any case, I agree that it's hard to explain computer stuff to non-computer people. That does not mean one can then be arrogant toward others.