Why does George say that migrant workers who travel from farm to farm are the loneliest people in the world?

Basically because they have nobody that cares about them. They travel from job to job alone. They meet people at the farm or ranch but people mostly keep to themselves. They are always afraid of being robed or compromised by somebody. Loneliness and isolation is a major theme in this story. Certainly Steinbeck knew the lives of these itinerant workers were lonely affairs. Men generally travelled alone and had nobody to talk to.