I think the fact that the main point of most games boils down to simply "play" is a major factor in why most aren't considered works of art. That and the fact that, like Ebert has described, games usually do not present a clear message or authorial point of view. Something like Psychonauts, imo, is the exception to the rule, where even though it was worked on by a large number of people it still had a very consistent point of view and clear authorial style -- Tim Schafer's. Something like Grand Theft Auto, while containing artistic touches (typically critiques on contemporary America, and largely on the media) don't really do it in a way that couldn't be done in any other medium. A hysterical parody of a typical AM radio talk show is still a musical arrangement, in some form or fashion.
