We've discussed what the Comfort Zone is in the
Pursue your Passion post where one has a comfortable job that brings in the money which pays for the basic human needs such as food and shelter (weekly groceries, bills and mortgage/rent). Many folks in the Comfort Zone know that they don't want to be there but are afraid that any movements will not only jeopardize their current situation but also introduce obstacles.
Most humans act on necessity and if there is no reason to change things then most of us leave things as they are. If the basic human needs of food and shelter are being met then why change things?