I agree that it's like MANY other social events, but not ANY.
The barbecue analogy is a prime example of an even it is NOT like. A basketball game (that requires a certain minimum number of people) is a closer analogy. This is a "team game" to a degree...to the point that not enough people really means a poor or nonexistent game.
For DMs, I think there is (should) be a higher standard, as should there be for the host of a barbecue. (If that person cancels, the whole event is shot). This is similarly true if, for some reason, a player and not the DM is hosting the game at their house....in which case there are really two hosts.
For players, it depends on the type of game. If a story rich game driven by character background and plot, not having a player really detracts from the fun of all. If it's hack and slash, and the player of the priest doesn't show, the others could be in for a tough time (even WITH DM adjustments).
I'll add that, unlike some other activities, D&D can require a lot of prep time/investment outside the game...time which then becomes wasted if the game is cancelled or a certain player does not show up. I am hard pressed to think of an appropriate comparison for this at the moment.
ALL THAT SAID: I absolutely don't hold gaming on some high pedestal, nor is it a grievous sin to cancel occasionally. I just think that many of the previous answers don't address some of the additional complexities in gaming that are not present in, say, a backyard barbecue.