^I asked that very same question to hubby last night. Why would they linger there, when there was no more food to be had?
We've seen with Morgan's wife that the zombies seem to retain some memories of their past lives (probably strong connections like family), as shown by her recognition of her own house after being some distance away, and repeated attempts to open the door. It's not enough to override the hunger impulse and prevent them from killing loved ones obviously, but they do retain some sort of unconscious connection.
Another example from the pilot would be the other deputy that Rick didn't like continuing to hang around the sheriff's office long after being turned.
The zombies milling about the school/aid center included soldiers and FEMA personnel who had manned it, so they make sense. But the others could include parents of students at the school (or teachers) or people who lost loved ones in the medical clinic.
The roaming herds on the freeways could be commuters, and their aimless walking could be because they're unconsciously continuing the journey homeward unless they come across a new food source.