Why do Star Wars collectors have to look like fat unkempt IT slobs?
Probably for the same reason that you just mapped "fat" & "unkempt" to people who work in IT. It's easier for people to laugh about someone the more they differ from the norm. We all have stereotypes of different kinds of people in our head, and comedians exaggerate the differences to get a laugh. It wouldn't be as "fun" to laugh at a happily married guy in a suit talking about his collection.
Besides, they're not all fat and unkempt. Take Weird Paul
for example. He starts slow, but it gets pretty funny in the last half.