because then you couldn't have all those dramatic shots of them broodily looking over a field of zombies. Or driving by them when they enter the prison.
I was thinking in this episode those field zombies are pretty damn lazy. Daryl slowly cruises by them on his bike and they're just like "eh." Then again, his Aura of Awesomeness is protecting him anyway. Nor do they ever try to storm inner gate whenever it opens. They (the survivors) don't even try to distract them away from the gate anymore.
I kind of wish the collar stayed on Andrea's Michonne'd zombie and you see him walking around the field dragging it around every episode.