I think you're forgetting a few variables in your analysis of the popularity of them. The kind of people that play FPSs to the exclusion of all else are not the kind of people that bought all those different kinds of games in the past. The audience for video games has vastly expanded, and I would argue that there's probably still around the same amount of people, if not more that like other genres. For example, in the 10 best selling games of 2012, 4 of them are FPSs - two CoD games, Halo 4, and Borderlands 2 (which doesn't quite fit into your war-type of FPS so it's questionable to being with). Outside of that, we have 3 sports games, 2 action games, and 1 rhythm game. There's certainly variety there.
As far as why FPSs are gripping, part of it is the skill involved and the ability to just pick it up and play. When you load into a game, you're the same as everyone else, and the only way to win is to be better. Not lucky or high level, just better. A lot of people see FPSs as the epitome of skill-based games. There's also the multiplayer; just like Facebook, if all your friends are playing this game and no other game and you want to play with them, you're going to pick it up too, right?