OK, its like this:
1. All those gamer stereotypes are true for a certain minority of the gamer population. . . a certain, very visible, portion of the gamer population. The people who don't fit the mold tend to "fit in" and often don't mention their gaming hobby because they don't want to be associated with the other guy. That out of sight, out of mind behavior just re-enforces the idea that the vocal minority is a representative sample.
2. Any hobby looks silly from the outside. Vampire LARPers dress in goth costumes and play rock, paper scissors while pining. Boffer LARPers dress like pleather knights and hit each other with vaguely sword shaped pillows. Model railroaders are grown men spending hours in the basement playing with toy trains. Look at quilting, skateboarding, free running, scrapbooking, martial arts or anything else from an outside perspective and the behavior involved seems a bit silly. In the case of gaming (and fandom in general), though, the vocal minority seems to lack the perspective that discussion of one's hobbies and/or interests is not always appropriate in a majority of situations.