Of course video games are art. In terms of purpose, they're no different than books or movies - they are forms of creative expression that exist to entertain people. To say that only two of those three are art requires one to define "art" in a way that specifically excludes interactive media. The going assumption seems to be that books and movies are pure mediums, allowing the author to deliver their vision to the audience unhindered by the obligations of others, while video games are inherently enslaved by the need to provide players with cool toys to play with, and even a narrative on par with the works of Stanley Kubrick would need to sacrifice its artistic merit for the sake of totally awesome action setpieces.
My problem with this argument, besides that it basically suggests video games are incapable of carrying an engaging fiction, is that it assumes that narrative is the only true form of art. To be art, so the conventional wisdom goes, a video game must be written and paced and played out in sequence exactly as though it were a movie, and a truly artistic game puts the player's interactions with it entirely at the mercy of the tightly-structured story laid out by the artist behind it all. Essentially, a truly artistic game must shun the trappings of its own medium, and become little more than a movie where players must occasionally press X to dodge.
Hardly anyone seems to feel, as I do, that the real artistry in video games is in engaging the player on a purely interactive level. Just as movies can thrill people by showing them worlds and characters that invigorate their imaginations, video games entertain people by placing those things at their fingertips and giving them the reins. To create worlds that people enjoy exploring, and characters that people enjoying playing with, takes talent, creativity, and vision. Just as we celebrate Quentin Tarantino for creating wonderful movies that millions have seen and adored, we celebrate Shigeru Miyamoto for creating truly delightful video games that have entertained millions of players. They are both artists - they simply chose to engage their audiences in different ways.