It's because the people who write the shows boil people down to the popular stereotypes. Same reason how every show about high school has everyone having wild sex and drinking all the time. The middle-aged men and women who write the shows think this is what goes on with the group, so they push it forward. The lowest common denominator plays well.
Nah. It goes beyond making stereotypes. Far beyond. With stereotypes, you make a bold generalization of something. With this, they're making broad generalizations to pretend to understand a counter culture. And quite frankly, they were more accurate with nerd culture when they were using stereotypes. Simpsons got Comic Book Guys and Nerds 2 decades ago. CBG first appeared in 1991 (and was more accurate back then), and the nerd trio (accurate to the point of quoting Monty Python to boot) came out in 1993.
Now nerd somehow means "plays violent mainstream game," (halo or GTA), "sees mainstream comic book/adult oriented nostalgia movies" and "probably read a mainstream comic book that one time." Maybe added "watches one mainstream current anime."
And yet, we get backhanded characterizations and every episode of everything goes to comic-con. Like they even know what that is. Heck, the 1990's Casper cartoon did Comic Cons with the harshest and most realistic light there is. There was even an episode of Freakazoid where he was sitting at a CC panel, where he kept getting questions about Batman TAS, until he snaps and says, "I dunno! Ask Paul Dini!" Nerdiest joke I've ever heard. The only current (ish) show that got comic nerds down pat was Batman Brave and the Bold. Not only do they constantly mock those who whine about how much they hate the show, they have the character of Batmite.
Nerds can be nerds for different things, and different sections of those things. Not all of them are obsessed with the exact same thing. And where's the shout out to animation geeks? Tiny Toons was doing that.