I already pointed out in What do we say when we say "this is art"? why is it valid to say videogame development is an art discipline, even though we don't really know exactly what an art discipline is.
The reasoning is simple: if someone says videogames aren't art, and she does it with the intention of devaluating them as less than cultural products of creative human work, then it's alright to counter that underestimation by saying games are in fact art, even if you have to forget the debate about the definition of art for a second. That is, saying "videogames are art" as short for "your underestimation of the interactive medium is wrong".
Thinking a little more about it, though, I found a new reason, that I now believe is more convincing, to say games are art without worrying too much about the exact definition of the word. And it goes: if we start with the assumption that music, cinematography, theater, literature, painting and sculpture are all art disciplines, then we have to accept videogames are just as similar and/or different from all those disciplines, as those disciplines are between each other.
Simpler: a sculpture and a song are such dissimilar objects, that everything they can possibly have in common they necessarily have it in common with a videogame too.
It may not sound that convincing, but the central idea is that for the moment it doesn't matter how do we define art, because any possible definition of the word includes videogames too, except only those arbitrarily designed to exclude them.
