A game isn't art just because It's good. It's art when the creator intended it to be art (this is not a waterproof rule, but in most cases this is true). If the intention is purely to entertain, then it's entertainment. If the intention is to invoke feelings or thoughts in the player, then it's more towards art. The focus of art is not to entertain, but to spread and invoke thoughts and ideas.
Games as an artistic medium is so young compared to other artforms. there's LOTS of new ground to break. Unlike many artistic areas, everything hasn't been done yet, Far from it! Even after decades of gaming tradition and a huge amount of technical improvements opening up doors, most games still follow the exact same concepts as the first generation of games. Some day, when games are picked up by artists, not geeks, games are going to take a whole new direction. This will probably happen around the time when art schools start to accept game devs, educate them, and push them to do artistic research.
Art as a concept is an extremely hard word to understand or define. People with artistic background constantly debate it with each other, people without artistic background usually have no idea what it means.
Since I'm pretty sure that Tetris or Mario Cart was made purely to be entertaining (really REALLY entertaining), I don't see them as art. They are really good games, but chess is a damn good game as well, but it's not designed to express anything or make you think. Deus Ex on the other hand has some political and philosophical thoughts on a whole different level, and is to me just as much art as mona lisa.
I've been studying dance and contemporary circus for five years now, and circus, just as games, is also traditionally seen as entertainment, but has broken into the artworld and is now a widly accepted artform. There's a clear line between contemporary and traditional circus, and while
is great entertainment, It's not made to be art, like for example
is. So far, most games being made fall in the same category as the chinese acrobats. Great entertainment. Little thought.