He is simply bald since the day he was born. Caillou was initially created as a baby of nine months. When it was time for him to get older, the addition of hair made him unrecognizable. So it was decided that Caillou would never have any hair, and he went on to become famous as a bald little boy.

Does Caillou deserve hair?

Another, now-viral quote by Arian Foster explained it well: “Caillou can't grow hair, not because he has cancer or progeria, but because he sucks, and even his own body recognizes that he does not deserve hair or food or love.”

Why is Caillou a whiny brat?

The reason for this is simple: Caillou is a shameless whiner. Virtually every episode of the show focuses on Caillou whining his way through a mundane task without reprimand or consequence. For lengthy segments, in fact, whining can constitute Caillou's only interaction with the surrounding world.