The definition of art, according to Brittanica...
"the use of skill and imagination in the creation of aesthetic objects, environments, or experiences that can be shared with others".
Now if that doesn't include videogames then nothing qualifies as art - that definition kinda puts all videogames into the art bracket.
I think though, the term should be reserved for games that are designed to affect emotion, not just look pretty. A lot of these 'art' games are simply pretty, and that's not enough. I mean, what games sparked the most emotion in you?
Braid? - give me a break, that game is so bloody full of itself. It's a decent platform puzzler, it didn't make me think, or react - I played it and tried to complete it (and failed), but for the challenge, not because I felt any sort of sympathy or empathy towards the little geography teacher dude. Now, Harvest Moon - Wonderful Life, that game made me react emotionally more than anything else, but does that make it an art game?... probably not, going by the general trend of art games. If Harvest Moon had, say the same aesthetic as Alice, or Journey, then it would be a different story - which just cheapens the whole term 'Art game'.
So all videogames are art by definition, but if we're strict with that, and we say that emotional response must be the main goal, then a lot of art games aren't art at all, they're just pretty. Personally I'm glad that indi developers are making games like Journey in the same way as I'm glad there are games like Binding of Isaac, too many indi developers are trying to make the next Angry Birds, trying to make something unique enough to make money... I'm glad there are still indi's out there taking a risk, and making a statement, before they necesserily make money.