"Adventure" and "socializing" (which is what I voted) plus "intellectual challenge" for lack of a better descriptor -- I like solving puzzles and problems, working out effective combat tactics, managing resources, investigation (clue gathering and mystery solving) and so on. I can get all three of these things by themselves just as well or better from other activities -- adventure from reading a book or watching a movie, socializing from hanging out with friends casually, and intellectual challenge by playing other types of games (boardgames, strategy games, etc.), and sometimes I can get two of the three -- some CRPGs combine adventure and intellectual challenge, playing non-rpgs with a group can combine socializing and intellectual challenge, watching a movie with a group of friends combines socializing and adventure -- but rpgs are the only (or at least the best) way I know of to combine all three.
The fourth reason (which is curiously absent from your list) is "creative outlet" -- method-acting, storytelling, world-building. I rather assume for many people this is one of, if not THE most important factor. I actually consider it something of a pitfall in that lots of people let it overshadow the other three to the detriment of the game (like any other factor, IMO, if this is your only concern you'd be better off with a different activity -- writing, in this case; the magic of rpgs is the way they synethesize so many disparate factors), but it's still definitely a factor, if not (at least for me) the factor.