So, we end up with no reason at all---it just happens, so on that note for both character and viewer, there's not much hope or answer, other than to keep running...and how long can that go on before it becomes stale, or stuck in repeat mode of temporary safety, infighting, external threats, and a shock zombie attack on a main cast member?
If you are watching this show for the zombie plot then I would agree, but TWD only uses the zombie apocalypse setting as a backdrop to the main story. The main story is the interaction between the main characters and how they react when faced with the end of the world. This is why the show is so popular, not because of the zombies (although that what might have gotten people in the door).
Like the best of Star Trek, this show rises above its own premise to be a thoroughly human drama.
The show could easily have been about survivors on a deserted island or have been placed after a nuclear holocaust, or even people stranded on another world or in the distant past. If you were to have taken the quality of the writing in TWD and given it to Terra Nova then that show would have been the success and TWD would have been cancelled after its first few episodes.
So, yes, if you are waiting for plot developments that surprise you and the characters are not your main reason for watching then I would suggest finding another show than interests you--there are many out there.