[icebraining]

...I actually wonder why they don't just pretend cell phones don't exist.

Because it feels forced. Ghosts and zombies are the premise of the stories, a thought experiment around which the plot unfolds. Pretending cellphones just don't exist feels like a contrived fix to the writer's inability to write a good plot. It's equivalent to a deus ex machina. Unless, of course, you can write a plot that hinges on the lack of cellphones - that could be interesting.