Birds, I can understand. But Goofy wears pants because male canines have hanging genitalia. The same goes for Equines, Bovines, Lupines, Felines, etc.
Girls aren't nearly as tempted to get a quick feel of a guy's bear chest as guys are to get a quick grope on a girl. Then again, maybe our society is just sex-starved... oh my god I can't say that with a straight face.
I've long considered cartoon characters to be complete imbeciles. He wears the towel because he's too stupid to realize he doesn't need it.