Hussar
Legend
/snip
In some ways, the real question might be "how are RPGs this popular in the first place?" considering all the competing hobbies, time investments and coordination issues, cooperative nature, lack of monetary rewards (as opposed to a poker group/fantasy sports things), need for a dedicated referee player, etc. What an amazing idea to hang on in spite of everything.
See, now there's an interesting point. The examples of hobbies that you bring up have all come in since RPG's became popular. Texas Hold 'em Poker, Fantasy Sports leagues etc. are all pretty recent compared to D&D.
So, what competition did D&D have in the hobby market in the mid-70's, early '80's? I would argue that there was a whole lot less then than there is now.