You need people conducting basic services in a city. If your characters need to buy an X, or you have a great scene planned requiring a store and the apocalypse you're going to need Apathetic Citizens to run it. If store owners and people followed realism and closed up their shops as alien spaceships you lose some story potential.
Some stories need Red Shirts. What better people to kill off than those nameless people who don't believe or don't care?