But there's always new stuff coming out for existing games, and that might be even more satisfying. There's something comforting about having all sorts of high-level perks to fall back upon in your game of choice, things you can't accomplish unless you stick with the game for a few years and learn its nuances. Truly embedding yourself in a game is a wonderful feeling, and it means that you always have a home to come back to (unless you get stuck with a shutdown notice, but that's another topic).
Absolutely none of us is immune to the siren call of a new game or the familiarity of an older one, but that's not the point. Given the option, do you generally look forward to the next title? Or do you find a game, settle in, and stick with it as long as it's operational?