You have to keep in mind that gaming is a very new medium. The more artistic variety of games has only been around for a few years. To demand something on the level of Shakespeare is very disingenuous.

A better way of seeing why games can be art is through a comparison to movies. Games can be made to essentially be movies but with the crucial difference of immersing the viewer entirely into the world through the choices it presents.

I do not see how adding more possibilities to an existing art form (movies) can ever demote something from art to non art (whatever you think these things mean).

I think a game like last of us is a great exploration of the potential of this form of game play.