And it allows time for any "knowledge transfer" that may need to occur.
i have been working in the valley since 96 as programmer and i never gotten interview or at the least needed help from people i work before.
That being said, the kind of person that tries to browbeat you into working an extra month with threats isn't a person that will be helpful in the future. Some bosses burn bridges when their employees have the audacity to quit. So don't worry about those kinds of people.
I've won places, and declined places, solely on the basis of small-city syndrome, where everybody knows each other.
People talk, so I've heard about conditions in a lot of shops I've never personally set foot inside. That's been enough to make me ignore their contacts, or adverts.