Your argument backfires here.
You just claimed that medium doesn't determine art. Put another way, (as others here have said) the medium itself is not art (to the other writer above, the pencil and the watermelon are not art, it is their combination that is the art). Similarly, video games themselves are not art. Not because they have objectives, but because what makes a video game a video game is not art.
Put another way, books are not art (they are blocks of paper stained with ink), but literature is.
Once again, as you said above, art is defined by the aesthetic experience. It is not only the medium, but what you experience through the medium that defines art. This is not to say that art is completely internal. It is to say that art is the combination of the paint on the canvas or the shape of the statue, combined with your experience of it.
And a "good" video game can be deemed as such only by the quality of experience (aesthetic only being one of them) you get out of it.
All that being said, that is all just philosophical precision stuff. As for Ebert - he's just bitter. From my definition, film isn't art either (but individual movies/ photographs combined with your aesthetic experiences of them can be).