Because it's a comic. Comics involve both illustrations of characters and those characters speaking, so he draws characters instead of just text. If you follow

Because it's a comic. Comics involve both illustrations of characters and those characters speaking, so he draws characters instead of just text. If you follow SMBC this is a pretty normal setup for him, and many of the comics are the same panels with different text. This happens constantly, with the most notable example being two girls hiking up a snowy mountain. He's not quite as egregious about it as Dinosaur Comics, where literally every strip is the same with different text. This comic was posted here because the text was particularly interesting this time. And it's not just talking heads, he also gives the illustration of the intersection as another visual metaphor.