Probably due to the general badassery of the remaining humans...just sayin'.
In all seriousness though, I have not been threatened yet by a zombie (Although Sam got somewhat close the other day). Like previously mentioned, I've walked right by zombies and made eye contact and had them just walk away as if it weren't worth the effort. That being said, probably not worth the effort to try and kill me
Time for a suicide mission!!
-Carry a 'generator' (duffel bag full of bricks, we're talking heavy) across campus