Because they compact revelations into a glance.  This one’s called “How it Works”:

This comic is more eloquent than any treatise I can write on the power of stereotypes (and of sexism in particular). I’m thinking of how many times someone’s annoyed me with their going 45mph down the 405.  As I pass them, I glance angrily at who’s driving, and on those occasions that they’re elderly drivers, I feel some kind of perverse rush at having my own ageism confirmed (obviously, I need to work against this bias).

Lest any of you doubt xkcd’s feminist cred, here’s the middle panel from another strip:

Ok, so that reads more like a blog post.  But generally, comic strips contain pithy epiphanies into reality.