Unless I missed it, there is no causal studies showing porn leads to predictable, verifiable negative health risks. We don't allow minors to buy that either.
And anyone trying to get such laws void would be doing the equivalent of political suicide. Most of our porn legistlation has their roots going way back into the 1800s which makes it very difficult to undo since we have been dealing with them for so long. That doesn't make those laws right by any means, and it doesn't justify the current crusades against video games.
Video games are much newer and gets the same protection that has been extended to music, television, film, and books since it is a recognized media for speech (as much as certain parties would tell you otherwise) of which its predicessors have already fought long and hard for their own 1st Amendment protections.