Because a disproportionate number of TV and film writers live and work in New York City. New Yorkers come to NJ and get annoyed by jughandles and driving on the Turnpike, develop hatred for the state, and then write jokes about New Jersians being shitty into their shows. People who've never been to NJ hear those jokes. If they ever do come to the state they're primed to look for bad driving and if they see an example even once while here they say "Ah just as I suspected!" and perpetuate the cycle.