Theater has been around for a long time, going from a way to tell stories to a complete art form of literacy. Improvisation theater, as a derivative of theater, is also considered as an art form, but requiring different skills : less memory, but more impulse. RPG have been created in the second half of the twentieth century with “Chain mail”. This genre of games have evolved since then, but the base idea : playing a character in a more or less realistic world, still remained.
What are the things that RPGs and improvisation theater have in common ? First of all, both activities require to play a character. Secondly, you cannot prepare what you want to do. Okay, in some games you can prepare some stuff, but generally, you don't prepare your character to do this or that, and just improvise with the unfolding of the scenario. Thirdly, there are more than one actors. Again, talking about things in general. Finally, the goal of both activities is to entertain each other.
Things sure seem similar. However there are many things that differentiate improvisation theater from RPGs : First of all, there are rules. Secondly, there are spectators in improvisation theater. Thirdly, there is a game master in RPGs there to control and make sure that everyone. Lastly, there can be an emotional link from the player towards his character since a player can play a character numerous times.
From this simple list, there is one major aspect that is CRITICAL to every art and which is absent from RPGs : the notion of spectators. Even in LARPing, gamers play for themselves, and the notion of spectators is an alien concept. When around a table, it is best if the players are oblivious to the surrounding world, to avoid being disturbed while playing. Spectators can still watch players, but the interest is quite limited. Unlike in improvisation theater, a spectator will rarely find much fun in watching RPG players. However, anyone can join a game, but one has to know the rules before playing, making the process quite unuser-friendly.
This is why I believe that pen&paper RPGs cannot be classified as an art form even though there are a lot of similitude with improvisation theater. Every art such as paintings, sculptures, music, books, theater... take the spectator into account, and that concept of spectator is crucial to be considered as an art.
Wow, this was quicker than I thought it would be. I am sure that many will disagree with what I wrote, and I probably forgot a lot of things, but I don't pretend to be a literary guy, quite the contrary ! But I like these types of “philosophical” debate-like themes, so I enjoy writing them. I hope that you took as much enjoyment in reading my articles !