IRC is simply a reduced form of communication where the only information passed from one person to another is that which they choose to transmit. On the other hand, when communicating with someone in person the information they are choosing to pass to you is accompanied by many layers of subtle nuances.

Next time you are talking to someone in real life and the conversation ends, notice how they will punctuate it with some form of body language. Look at them carefully and think about how they shrug their shoulders, give a smile or maybe briefly exhale through their nose. See if their eyebrows rise or if they look right at you or maybe look down at their shoes.

Compare that with looking at a screen and noticing that those little words have stopped appearing.