Why only Garfield ?

Understand something: Screenwriters write roles. This means that while there are often descriptions of objects and settings in movie scripts, most are explicitly discouraged from putting in descriptions of the characters. They WANT to be able to offer the same role to Harrison Ford, to Will Smith, and to Viggo Mortenson. Thus there is NO WAY that Jon is going to look like Jon in the comic. As mentioned above, they chose to go with a real dog for Odie. This, to me, is one argument against computer graphics, of course. Garfield is stylized the way it is for a good reason. Nevertheless they chose to incorporate live actors, and that means they want to not care what the characters look like -- so they have the broadest variety of comedic, in this case, talents they can offer it to.