Despite its blithe dismissal, I think the explanation below (via Paul Bausch) is exactly why nothing akin to tabletop roleplaying games (notwithstanding Kriegsspiel) appeared before the modern era. Most of history is forgotten, and surely a historian would know this.
An interesting variant of the argument, suggested by Matt Clancy, is that in fact tabletop role-playing games have been invented and re-invented many times, all over the world, but because of the lack of printing and low population densities, they have become lost and forgotten. Perhaps. Though I find it hard to believe that an activity so fun would never have been mentioned.