AIM Conversations and Endless Pauses

Imagine for a moment you are having a serious face-to-face conversation with a friend sitting on couches in your living room. Mid-sentence, he stands, walks out of your house and drives away. You sit, staring at the couch where he was sitting, waiting for some sort of closure. After perhaps five minutes, you go and find something else to do, perhaps a bit miffed. Two hours later, your door opens, your friend walks in, sits on your couch and finishes the sentence he started two hours before, not caring whether you're sitting there or not.

Why do we let it happen online but not in real life?