Sure, you cannot buy a licensed mug with Hobbes on it like you can with Garfield or Snoopy, but Watterson has surely made millions on the many book collections of his work. And book volumes are certainly merchandising.
Absolutely, he has given up the lucrative money that other cartoonists like Jim Davis have achieved by licensing/merchandising anything/anyhow, and still more by ending the comic after 'only' 10 years, but it's probably an easier decision to forgo the tackiness when the books have done as well as they have.